@JimmyFromTheBay

I've installed the MESS fork for it (lr-mess-mz1500-cass).

What is the MESS fork ?
Please give a better explanation on how you installed.

With this command you can find the supported extensions :

pi@raspberrypi:~ $ /opt/retropie/emulators/mame/mame -listmedia mz1500 SYSTEM MEDIA NAME (brief) IMAGE FILE EXTENSIONS SUPPORTED ---------------- --------------------------- ------------------------------- mz1500 cassette (cass) .wav .m12 .mzf .mzt

These are the loading instructions for MZ-1500 :
Hit C key
TAB > Tape control > Play

I used cassette files inside mz800_cass.zip from mame-sl.
mucmac.zip worked for me.