I posted this back in January but did not receive a solution so I'm posting it again and asking if anyone is having the same problem.
Here's what happens when I:
1. Open any song
2. Right click on any track except Drums
3. In the dropdown menu, click on Select Hi-Q MIDI Synth patch OR Select MIDI Synth or Audio Plugin
4. Select any instrument in the list box
5. Click the Play button. I get a Message box that shows the file path as C:\bb\DX Settings\(file name).tgs OR C:\bb\DX Settings\Amplitube\(file name).tgs
6. Click OK on the Message box
7. Get an error message that says there is no audio demo available for the file followed by the file path/name.
Please try this and let me know what happens for you. I'm using BIAB 2015 build 415. Thanks a bunch!

Hi rpuff

I can confirm the issue you are experiencing.
Maybe send the issue to PGM support?

VideoTrack,

Thanks for your response. I just sent this issue to Support.

Reply from PG Support:

I spoke with one of our developers about this and he explained that the program is looking for an audio file with the same name as the TGS file but with any audio extension ('.WAV','.WMA','.M4A','.MP3','.AIF','.AIFF','.MP4').

However, these demos have not been made yet which is why you are getting that error message. They are hoping to have these demos available to release with a future patch.