Hi
Since installing RB 2009.5 I have found the following occurs consistently.

Start Play and while the song is playing, select another track in the track window - if you select the track on the Track Number (checkbox) the sound drops away. If you select the track in the grey bar (next to the number) it does not. When the sound drops away and you press the stop button and then start again, everything's back to normal.

I now just don't select a track by clicking on it's number but on the bar - but it may be a bug?

By the way where do we report suspected bugs on the PG site?
Thanks
Roy