For anyone having problems with mp3s that do not play, use this mp3 and see if it works. If it does, then your mp3s are not encoded correctly.
Eric Olson has graciously given us permission to use his song with this plugin.
Unzip into:
HL: <Steam Dir>\SteamApps\<Username>\half-life\valve\sound\misc\loading\
DMC: <Steam Dir>\SteamApps\<Username>\deathmatch classic\dmc\sound\misc\loading\
CS: <Steam Dir>\SteamApps\<Username>\counter-strike\cstrike\sound\misc\loading\
Play using console command:
mp3 play sound/misc/loading/chances_remix