Well there are two ways :
client_cmd(id,"speak hello") which will just use CS SOUNDS "
http://www.adminmod.org/help/online/...ife_Sounds.htm" A list of all sounds default of cs.
or
Precache the sound, add the :
client_cmd(id,"mp3 play sound\backstats\song1.mp3" -> .mp3 format
Do not forget to upload the song to server
To run an plugin well, i just know one thing, you can an register command, on the plugin you want to run, than on other plugin just use client_cmd(id,"backstats") but u have to add this on the plugin u want to run :register_clcmd("backstats","Function")
Hope i didn't confused you and helped you much
Regards
EDITED :
ALSO THERE'S ANOTHER WAY !! You can reproduce the song without using mp3 play, i don't know how, but is possible i saw that, maybe someone can answer you about this i said on this little 2 lines.