Make sure 'Send most recent Patch/Wheel/Controller' is enabled in the MIDI Out preferences dialogue.

You may be hearing the audio track at the volume of the last received volume message instead of the one you currently desire.
In other words, if the last volume message was 20, and you stop and rewind to where the volume should be at 120, but start playback at a point after the message to change it to 120 is sent, it may still have 20 as the current volume unless this is feature is enabled.
This makes it look back and see what the previous controller messages where before playback.

(yes I understand these are audio tracks, but the volume adjustments themselves are MIDI data)

Make your sound your own!
.. I do not work here, but the benefits are still awesome