Convert samples to 16bit 22.050kHz PCM or lower, maybe. Load them into milkytracker and set the sample quality to 8bit.
Or grab some samples from this little gem: http://www.bluemoon.ee/history/scwin/ (Piano roll sample-based module sequencer). If you Insert Voice then write a note and export the song as wav the quality is better than if you export the Instrument as wav from the "View>Instruments" screen.
Boy Scout is a PC-based tracker for writing GBA songs. I have not figured it out yet, so I cannot really comment on the ease of use, etc
It only uses the DMG Legacy channels, so not what OP is looking for.