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.