Soundtrakker 128 module
From Just Solve the File Format Problem
.128 files are modules created by Soundtrakker 128, one of the most popular music tools on the Amstrad CPC. Soundtrakker 128 is (in most cases) compatible with SNG files produced by Soundtrakker v1.0 and v1.1, although it may not play them correctly when very short patterns are used.
- Soundtrakker 128 (Commercial)
- Soundtrakker 128 (Freeware)
-  -- this discussion links to a now-dead site allegedly containing the Soundtrakker source code. Might be worth trying to track that down if it's available.