How to reproduce:
1. Open attached "AudioIssue"project and build it on Android
2. Turn on the Oculus Home -> Settings -> See All --> Do Not Disturb
3. Launch application on GearVR, notice the sound playing
4. Make a call to the device running the app

Expected result: Sound is still played since Do Not Disturb is enabled
Actual result: Sound stops playing during a call

Reproducible with - 2018.2.0a7, 2018.1.0b13, 2017.4.1f1

DUT:
Samsung SM G950F*, OS:8.0.0,CPU:arm64-v8a, GPU:Mali-G71,Build:samsung/dreamltexx/dreamlte:8.0.0/R16NW/G950FXXU1CRB7:user/release-keys
Samsung SM G950U*, OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 540, Build:samsung/dreamqltesq/dreamqltesq:7.0/NRD90M/G950USQU1AQC8:user/release-keys
Samsung SM G928F*, OS:7.0, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zenltexx/zenlte:7.0/NRD90M/G928FXXU3CQG2:user/release-keys

Won't fix - Testing with other apps made with other platforms has proven that the behavior repro's regardless of the engine used. This has been bugged back with Oculus itself, since it appears that they are probably not fully handling the "Do Not Disturb" settings during incoming calls on GearVR.