Personally, I'm interested in programming for the sound chip; maintaining multiple harmonies with varying ADSR and noise to try to get as much depth as possible.
I'm not a low-level developer, so my progress thus far trying to fit an experiment-able design into 6502 machine code has been pretty slow going. Any chance to get closer to C syntax, real variables names and nested structure is a welcome relief to me. Thanks for mentioning cc65, I had not discovered it yet.
cc65 will certainly scratch the itch you've got. Since control of the sound chip doesn't require super fast code, the significant overhead of running C on a 6502 won't be an issue, and you'll be in a familiar environment.
However, there are also lots of music editors for the C64, both native and running on PCs, that can give you very fine grained control of the SID without programming. I believe GoatTracker is highly regarded, but there are lots.
I'm not a low-level developer, so my progress thus far trying to fit an experiment-able design into 6502 machine code has been pretty slow going. Any chance to get closer to C syntax, real variables names and nested structure is a welcome relief to me. Thanks for mentioning cc65, I had not discovered it yet.