If you are in a Mix Project there will be grey boxes in between your voice clips, those grey boxes represent music breaks in your message.
To adjust the time click on the three dots(...) and select your desired break time from 0-60 seconds. This can be applied to all or just one of the breaks.
If you are in an AI On-Hold Message Project, there will be "Start Music" "Music Breaks" and "End Music"
To adjust you will see the length of music that is defaulted, you can click on each one to change the music from 0-60 seconds.