Unlike standard Commodore 64 which lacked internal slots for expansion cards like modern PCs and Mac's.
Not needed. No one would want to have any other sound chip than the SID, the coolest sound chip ever built.
You could actually buy 'sound card' for standard Apple II that can produce better sounds. Then again, Apple IIGS had a dedicated built-in sound chip that is vastly superior to even Amiga!
Technically yes, but it had a crap mono output and while there were hundreds of thousands of cool tunes made on the Amiga there never was anything like the MOD scene for GS users.