Steps to reproduce:

1) Download attached project 'reproProject.zip' and open in Unity
2) Make sure Virtual Reality Supported is enabled and Virtual Reality SDK is Cardboard in the Player Settings
3) Build and Run 'test' scene on a device
4) Try to increase/decrease the sound with device's Volume buttons

Expected result: Volume buttons should increase/decrease the sound
Actual result: Sound stays the same

Reproduced with:
5.6.0b1, 5.6.3f1, 2017.1.0p3, 2017.2.0b6, 2017.3.0a2

Devices under testing:
Google Pixel, OS:7.1.2, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:google/sailfish/sailfish:7.1.2/NJH47D/4045516: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 G935F*, OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:samsung/hero2ltexx/hero2lte:7.0/NRD90M/G935FXXU1DQEP:user/release-keys

Note: Volume buttons works using Gear VR
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Fixed in:5.6.3p2, 2017.1.1p1, 2017.3.0a5