This is likely something that can be added via programming and not need any hardware changes:

If a 'sleep timer' is a part of the feature set, change the programming from

1) Wait X minutes
2) Power off

to

1) Wait X minutes
2) Start a loop with 60 (or 120) iterations
3) Volume down by one dB (or by .5 dB)
4) Wait 2 seconds (or 1 second)
5) End Loop
6) Power off

This way, instead of 'power off' suddenly cutting the sound, the volume will be reduced by 60dB slowly over two minutes, or even to zero if the volume was not that high to begin with, before the 'power off'.