I have the same problem.
I have Optifine installed on Minecraft 1.5.1 (installed after Forge for 1.5.1 version .600)
No sounds whatsoever.
No, my speakers work fine.
What is PaulsCode?
"Get a better archiver" is NOT helpful at all.
-Things should just work... what is wrong with the sound?
PS. I installed forge by putting the .jar files and folder things into the minecraft.jar