Hi!
You are right to both statements, there was essentially no volume control over the DPCM channel in the NES and yes, you need to use an external tool to convert from WAV to DMC (FamiTracker and RJDMC are examples.)
- In FamiTracker, if you select a sample and click "Edit" you can tweak it again.
- In RJDMC you can simply open a DMC file and tweak it and save again.
The amount of tweaking you can do is fairly limited, RJDMC can trim, change the volume and fade in/out, that's about it!
Im not 100% sur about this, but since DMC files are reduced to 1-bit DPCM, you might be loosing quality by constantly re modifying the DMC file (kind of like re-saving a JPG image multiple times). For best quality it is probably better to find the shortest path from WAV to DMC.
-Mat