Well, it looks like it hooks up via optical cable, so most any soundcard will do. I do note that by using Dolby/DTS formats you do take an audio quality hit though, so if they can connect via analog, then do that instead [might need to do a TRS to RCA conversion in that case though...]
In that price range, either the ASUS Xonar D2(X), HT Omega Claro+, and Auzentech Meridian 2G are your best options. Note that the digital output on some of these cards uses Coaxial instead of Optical, but the cards usually ship with a converter to Optical [I know this is the case with ASUS; not sure about the other two].
5.1 speakers are made for movies or games, you can hear music also, but music is on 2.0 channels, if you want music on 5.1 channels you'll need to use DDL or DTS connect, if you watch mkv movies, mostly of them are on dts 1536kbps or dolby 640kbps, both are supported on toslink bandwidth, games can sound better via analog connetion but unless you are an analog fan you don't gonna care about it.
if you want the maximum quality on analog output and spend more money you can go for HT OMEGA Claro Halo XT:
or if you find, an ASUS Xonar H6 for ASUS Essence ST/Xonar HDAV1.3, it might be discontinued.