Hmm yeah, that appears to be a side effect of exporting the song to mono, which causes some settings to be ignored when calculating the final sample volume. For the time being, you'll have to export a stereo WAV/FLAC file and convert that to mono (in OpenMPT's sample editor or any other external sample editor). This will definitely be fixed for OpenMPT 1.30, not sure if there will be a backport for OpenMPT 1.29 as this will also have impliciations for libopenmpt (and we only plan to have at most one more 1.29 release if all goes well anyway).