After setting music mode some tracks are silent when playing or exporting song

Hello all, I would like to change the tempo of my project which contains both MIDI and Audio tracks. For doing this I opened the pool and set for all audio tracks the music mode. When I play or export the song after this some tracks are silent without having changed anything besides setting the music mode. If I disable the music mode again I can hear these tracks again. What is the problem and how can I solve it? Thank you very much in advance for your help.