The C64 sound chip (!) is also memory-mapped, which tends to speed up operation quite a bit compared to I/O based solutions.
This assumes that the 6502 can access memory in a C64 faster than an Intel 8088 in an 5150. In the 5150, I/O is accessed just as fast or almost as fast as memory, since both busses are set to the same speed.
There was an extremely obscure sound card called the Innovation Sound Standard which mapped its SID's registers directly to 31 separate I/O addresses. That is a lot of addresses to use when the 5150 had only 1024 total that were to be used.