I've seen a large range of quality from sound cards. The issues seem to fall into the D/A converter stages, and amplifiers. Some of the on motherboard solutions (and no name sound cards) use very inexpensive D/A stages, and sound very poor when driven into good amps and speakers. Many integrated amps have excellent A/D conversion, allowing you to bypass most of the issues with inexpensive motherboard sound card solutions.

My home office uses a computer motherboard which happens to have both optical and coaxial digital audio outputs. I run this into my system... excellent sound quality. I refuse to buy the speaker / sub / amp setups most computer manufacturers or computer stores stock. They're generally of dubious quality for the $.

Another area I've seen issues is with the connection between PC and receiver. Some PC's have such a noisy environment, that a great deal of noise can be injected into the receiver over the shield of the digital coaxial connection. Look for a PC or sound card with a optical digital link. This totally bypasses any possibilty of ground based noise injection from the PC.

I've also used systems with Creative's sound card solutions. They tend to perform very well, and offer excellent performance at line levels. I'm not impressed by the performance of the amps in their solutions, but you're not going to use this, so not a concern. I know they just came out with a new line of cards, I bet you can find the last generation cards now at fire sale prices, and for mp3 playback, they will do a great job. (The new designs offer lots of improvement in the number of voices, midi support, etc... none of this means anything for your application, so save the $'s, and go ahead and buy the older generation cards.)

I think the bigger question is how do you plan to control the PC and playlists in the computer from the remote locations? Have you found something you feel does a good job? I'd like to hear what you plan to use.