How to reproduce:
1) Create and Audiosource
2) Add an AudioClip
3) Configure the Audiosource with a negative pitch value like -1 and enable play on awake
4) Press play. You will not hear the sound
5) Enable loop and play again. Audio will play
By Design: User shouldn't use negative pitch value, unless one wants the clip to play backwards in the loop. If user wants it to play once through backwards, firstly he/she needs to set the position with AudioSource.time to the end of the clip and then play it.