Reproduction steps:
1. Open the attached project " BeiJingPixel_UnloadUnusedAssets4.rar";
2. Open the Game.unity scene;
3. Enter the Play mode in the Editor;
4. Observe that sounds are playing correctly;
5. Stop the Play mode and enter it once again;
6. Observe that sounds are not playing.

Note: sometimes, the behavior in 3-6 steps is reversed, sounds are not playing for the first time.

Reproduced on: 5.3.5p6, 5.4.2.f2, 5.5.1f1, 5.6.0b4.
Reproduced both on Mac and Windows system.