MilkyTracker

MilkyTracker is an open source, multi-platform music application for creating .MOD and .XM tracker module files. It attempts to recreate the module replay and user experience of the popular DOS program Fasttracker II, with special playback modes available for improved Amiga ProTracker 2/3 compatibility.

High-resolution version for tablets: http://sites.google.com/site/xpelyax/Home/milkytracker-highres.apk
Hacker's Keyboard is recommended for high-res version: https://play.google.com/store/apps/details?id=org.pocketworkstation.pckeyboard

Watch the video to learn how to edit envelopes in the instrument editor - you need to touch the envelope marker, wait for a second, then quickly move finger to the side.
This is definitely tricky - if you will wait shorter the marker will not snap to the finger, if you will wait longer you will get popup menu and the marker will stay in place.

There are few demo songs pre-installed, just hit the "Load" button.