Being clueless to C-Media soundcards, I can only guess that the soundcard is forcing the CPU to do the work of decoding the MP3s, and possibly other sound-related tasks. Therefore, when you play your MP3s, you're making the CPU work. In most cases, the CPU has to work to play MP3s, but its possible that the new PCI card is more reliant on the CPU than the integrated solution that you had. Therefore, you need to do a few things. First, you need a new HSF. Period. Those temps, even at stock, are way too high, and I find it hard to believe that AMD supplied an underpowered HSF for their chip (though, it's quite possible, since it is in spec). Was that a retail CPU, or an OEM that came with a HSF? Regardless, you're killing your CPU more quickly than you need to.
Second, you need to check the case temps, as lowering that 3C will lower your CPU temps 3C. Use MBM 5 to check it out.
Last, you really should consider a different sound card. It sounds like, that while the sound quality increased, the reliance on the CPU is just too much. To see how bad it is, try running a benchmark with sound from the PCI card and sound from the integrated card. I would guess that the PCI sound card is sucking more CPU power than the onboard did, which would give you lower benchmarks. I would suggest either a Hercules Fortissimo II or Game TheaterXP (though that will run you $120-150), or a Turtle Beach Santa Cruz, or a Philips Acoustic Edge. Those three makers have some pretty nice audio solutions, and I promise you that the Game TheaterXP (and the FortissimoII, besides in Black&White) run perfectly, and have no problems with resource conflicts. I tend to shy away from the SB lines, since the one SBLive!Platinum I had caused problems, and they seem to have issues with VIA chipsets. I also shy away from C-Media solutions, since they suck a lot of CPU power for the quality you get.
As for the decoding of MP3s by WMP, you need to have a sound card that can do it (all that I named above list hardware decoding as existing). For the Hercules cards, it's a setting in the control panel that you use, telling the drivers to route all MP3s to the soundcard. But, at least for Hercules cards, this only works in WMP and other players that use the DirectShow aspect of DirectX, I don't think this works in Winamp. In fact, the only other that Hercules knows of is AudioStation by Voyetra/Turtle Beach. Perhaps Winamp 3 will use it.
The Phillips has probably the best directional sound of all of them, called QMSS. It is s'posed to be extraordinarily good at placing sounds, so you know where it's coming from.
The Turtle Beach is a good card, cheap, but has nothing that stands out. If anything, it's great competition for SB, as are the other three cards.
So yeah, that's my long-winded post that probably does nothing to help you. But, you really need a new HSF. Really. Especially if you're going to overclock. Invest in a Thermalright AX-7 with a moderate 80MM fan, something in the 45-55 CFM range, or ... well, I can't think of another right now. That will set you back 35-45 dollars, depending on shipping costs. Look at Plycon.com. They had the best prices that I found so far. But, your processor is running hotter, probably because the PCI sound card is forcing the CPU to handle more than it did before, upping the core temps.
-SammyBoy