Envelope + (Rapidly) varying tempo give undesirable output.

[PhysSong]

TempoNaN.mmp.zip

The tempo model is connected to a LFO controller. Instruments with envelope sounds weird, some notes may be skipped.
Edit: some notes may be skipped regardless of the envelope's status.
If arpeggio is enabled, it sounds more glitchy. Infs and/or NaNs are also generated.

[PhysSong]

NotePlayHandle::resize( const bpm_t _new_tempo ) changes m_totalFramesPlayed, so EnvTotalFrames and EnvReleaseBegin in InstrumentSoundShaping::processAudioBuffer() can have wrong values.
NotePlayHandle::resize() has introduced in 52d9ca4, which was done in 2007. I don't know why it exists and why it should be there.

[PhysSong]

That function is there because notes should be resized when the tempo changes. However, it shouldn't change m_totalFramesPlayed. Instead of this, left frames should be recalculated.
There are some exceptions, like notes from MIDI input or arpeggio sub-notes(if tempo-sync is disabled, at least). Creating a new variable m_shouldBeResized may be handy, in my opinion.

[zonkmachine]

This sounds to me like another case where it would be easier to troubleshoot/test if #2606 is fixed, as some of the symptoms are overlapping. Dropped notes etc.