I assume the following represents your current configuration.
[linksys](lan)<-- wire -->
(wan)[d-link]<-- wireless -->[mp101]
Let’s assume the Linksys is using the 192.168.1.x subnet, and the D-Link is using the 192.168.2.x subnet. From a security point of view, this configuration is basically worthless. It’s not subnetting that provides protection, it’s the firewall. The subnets are different only out of necessity, since you have two adjoining networks across a WAN port (it prevents ambiguity). In this case, the D-Link firewall is protecting the wireless subnet from the wired subnet, when in fact you probably want just the opposite. Anyone on the wireless subnet has access to ANYTHING upstream, both the 192.168.1.x subnet and the Internet. However, anyone on the wired subnet is prevented access to the wireless subnet, which doesn’t amount to much since it’s only protecting a little ol’ network radio.
Despite what I just said, you report the MP101 can NOT access shares on the Linksys subnet (not sure about that, it wasn't clear from your description, but let's assume that's the case). The only way that could be true is if you did NOT use the WAN port of the D-Link, but used a LAN port.
[linksys](lan)<-- wire -->
(lan)[d-link]<-- wireless -->[mp101]
In this case, the firewall is not even relevant since the firewall only applies to the WAN port of the D-Link. The reason the MP101 would NOT find the shares on the Linksys subnet in this case is because there’s no routing by the D-Link. When using the WAN port of the D-Link, the router knows where the 192.168.1.x subnet is located, and takes responsibility for routing users of its own subnet over the WAN port and to the promised land (the 192.168.1.x subnet). But when you connect them LAN to LAN, that doesn’t happen and the two subnets can’t see each other, at least not without some help.
If the Linksys and D-Link are connected LAN to LAN, it only requires someone on either subnet to manually reconfigure their computer for the other subnet to gain access (child’s play). So the fact you’re using two subnets provides no security at all. Even worse, if both DHCP servers are active (Linksys and D-Link), it’s pure luck which DHCP server responds first. And if the wrong one does, your clients will be misconfigured.
That’s a rather lengthy and tedious way of saying, it doesn’t really matter how the Linksys is connected to the D-Link. The real problem here is the weak security of the MP101 (WEP).
In general, as long as you use a modern wireless router w/ strong protocols and encryption (e.g., WPA/WPA2), and long, random passwords from a large character set ( e.g.,
http://grc.com/pass ), you’re not going to have problems w/ wireless security. NOBODY is going to crack into your wireless system when you use the right equipment, configure it properly, and use some common sense.
The D-Link only supports WPA (at least according to the manual). The OP should check w/ the manufacturer and see if there’s a firmware update that *might* have added WPA2 support. But no real harm if they didn’t. While I’d prefer WPA2, it’s not the end of the world, WPA is just fine too. So the D-Link is up to the task. However, the MP101 is another story. It only supports WEP (as far as I can tell from its manual), which is easily cracked. NOW you have a right to be concerned. It’s the weakest link in the chain. Here too, the OP should check w/ the manufacturer to see if WPA or WPA2 was added in a later firmware update. In all likelihood, it wasn’t given how old this device is, but it’s still worth checking.
So let’s assume things remain as they are, WPA for the D-Link, WEP for the MP101. What are the alternatives? Since the MP101 has an ethernet port, it’s possible to use a wireless ethernet bridge that supports WPA (all modern bridges do) and completely bypass the wireless of the MP101.
[linksys](lan)<-- wire -->(lan)[d-link]<-- wireless -->[wireless ethernet bridge]<-- wire -->[mp101]
Of course, this adds some expense. The bridge could run $20-30 depending on whether you can find a good deal. But it solves the problem. You’d connect the D-Link to your switch LAN to LAN (not the WAN), disable its DHCP server, and give it a static IP in the same subnet as your primary router (e.g., if the primary router is 192.168.1.1, perhaps make the D-Link 192.168.1.2). Now everything is on the same subnet, no additional firewalls, etc. The D-Link is just a simple WAP (wireless access point). And your wireless ethernet bridge connects to it over WPA in support of the MP101.
Another option is to run powerline adapters between the MP101 and the Linksys, eliminating the D-Link completely.
[linksys]<-- wire -->[powerline adapter #1]<-- power lines -->[powerline adapter #2]<-- wire -->[mp101]
In this case, your power lines become your bridging mechanism, not wireless. The only issue w/ powerline is that it doesn’t work in all situations due to variances in home wiring. And bandwidth isn’t always that great (might be as little as 2-3mbps in the worst case, more likely 10-12mbps for the average home, occasionally a little better for a lucky few). But since you’re only streaming audio, even 2-3mbps is plenty. And it’s simple to install and configure (no configuration at all if you don’t enable security). It’s possible (if improbable) your powerline signals might travel beyond your home, perhaps to a neighbor. So they usually come w/ an encryption option. Secured or not, it’s a lot safer than either open or WEP secured wireless.
Note that you’ll need a PAIR of powerline adapters.
Here again, you have to consider the cost. A pair of 85mbps powerline adapters is probably going to run $40-50, although you might find some refurbished ones from Newegg or Amazon for say, $30-35. But it’s still going to be an expense. That raises the question, is it worth pursing these bridging solutions, or just putting the money towards a better radio than the MP101?
That’s the problem w/ old technologies like the MP101; they end up costing too much to maintain as the years pass than if you simply updated to a radio w/ newer technologies. You not only simplify your network, but you get a better radio as well.
So it’s up to you. Using the D-Link with a different subnet doesn’t solve anything, no matter how you hook it up. As long as someone can get on your wireless network over WEP, they have access to your network. The only solution is to either use a bridge, or live w/ the insecure WEP protocol.