That's definitely neat, but the noise it picks up is unfortunate. I wonder if there's a way to reduce (filter?) that?

Yeah same. It sounds cool but the result is not very good.

Yea, that's what I figured. Reminds of people that do the stuff with LSDJ and multiple gameboys. Of course, the TI calcs have fewer channels, but same idea.

With this type of sound engine it's tricky, due to the way multi-channel pulse frequency modulation works. The main problem is that with PFM, you have a duty cycle ratio of  ~1:99, as opposed to 50:50 on pulse interleaving based routines such as CalcMOD. This makes the bass very quiet compared to the rest. Adding to this, the "TIM-1" engine used here was one of the first 1-bit routines I wrote, and has particularly thin sound.
In the end, both PFM and PI have their pros and cons. PI will always sound cleaner and bassier, but PFM allows for more channels, and offers more flexibility in terms of (fake) volume control. On ZX Spectrum, we now have a 16 channel sound routine

Anyway, you might be pleased to hear that after a long absence, I've returned to coding for TI calcs, and am working on a new sound routine. I also have plans for a major HoustonTracker rewrite, but I won't get around to that till the end of the year.