1 bit sound
Basically, the Spectrum makes 1-bit sound using bit 4 of port $FE. Beepola (and other software) uses that to generate sounds. MSX can do the same using bit 7 of port $AA (PPI). That was the thing.
Inside the ZIP
As said, Beepola has multiple music replay engines. The tune included on the zip file (tune3.bin) comes as example tune together with the Beepola tracker. This tune uses the Phaser1, Synth Drums beeper engine and was exported to bin and can be assembled together with the modified replayer (1bit.asm) to produce a combined binary file (1bit.bin) that can be loaded and make work on an MSX. Just use the MSX-Basic loader (1bit.bas).
The code comes with no warranty. It's old. It was just a quick test and it's not optimized, so probably not accurate. It uses the MSX PPI port to make the buzzer sound. PPI also controls the CAPS key Led, keyboard reading and cassette, so some bits have to be masked. So use it at your own risk!!
If you like 1-bit sounds, you can check the Beepola tracker at its own site: Beepola