Is there a way to verify this? I've also had that issue (can't reproduce) where the sounds don't start until after the first music play. Not sure how to make it happen again.
All my music is encoded as an OGG, all sounds are OGG+M4a.
To me I feel like the audio menu should be changed a little to differentiate these things.
Play Music (Tag): Stops all other music automatically, plays new music.
Play Sound (Tag): Just plays the sound.
This would remove the need to have "On end of Layout: End stop sounds.."