I have had pretty good luck with Audacity and the free plugins they offer.
You basically just use the tone generators, and "play it by ear".
If you own a C-64 still you can use an emulator like CCS64 or Vice-64. Then you can use the native SID emulation to get pretty close to the original sounding SID chips.
They use a old basic interpreter so generating sounds is pretty simple.
If you do not want to learn the old school style basic you can also use Audacity to "rip" sounds, so you have a reference.
Once you have a reference sound it is pretty easy to emulate them in audacity by generating tones, and speeding up or slowing them down.