vgmrips

I really wonder whether this pack should be mono or not.
Sometimes the stereo effects don't sound too bad, but some songs play completely on the left speaker.

It's how it plays in MAME. Is there something I can do, short of modifying all ES5503 register writes, to make it play in mono?

In the VGM header, there is a value "ES5503 output channels" at offset 0xD4. It's 2 by default (stereo). Setting it to 1 makes everything mono.
The actual question is though - how does this play on the actual machine?

I don't have an actual Apple IIgs myself, but I know that the normal audio output from an Apple IIgs is mono; a stereo expansion card is required for stereo output. Looking at the game's internal sound files, which are basically MIDI files, there are Program Change and Volume Controller events, but no Panpot Controller events. This indicates to me that the panning is probably an artifact of the MIDI player library that Sierra used --- possibly assigning pan positions by MIDI channel number, or having a fixed pan position for every instrument --- and that mono output is what was intended. If you agree, then I will reupload the pack modified in the manner that you described.

Something I forgot to mention (sorry): You can use vgm_sro on VGMs that upload their samples to sound chip RAM/ROM. It will remove all unused sample data.

Okay, but should I set channels to 1 or not?

Silence is approval, so I have reuploaded the pack with vgm_sro applied, and made the files play in mono.