Which p2p share client program are you using? LW, Bearshare, GNucleus, etc.
Sounds like you need codecs for your player. A list of
CODECs sources can be found here (choose one of these sources): 1.
Free-Codecs.com, 2.
Nimo’s codec pack, 3.
CoDecs & codec identifiers, 4.
K-Litecode Codec Pack
ALso consider getting a backup player with built-in codecs such as DivX or VLC:
http://www.videolan.org/vlc/ (choose nearest mirror site)