Steps to reproduce:

1) Download attached project 'TestingAndroidVideoFix.zip' and open in Unity
2) Make sure device is connected to network(Wi-Fi/3G/4G)
3) Build and run 'scene' on a device
-------Once app starts you should see video being played in short invervals
-------Also, in scene view there's button on the top left corner '[1] 3GPP (3gp5)'
4) After video finishes, start quickly tapping this '[1] 3GPP (3gp5)' button until you see some screen changes or hear audio
Note: There might be 3 outcomes after 4th step :
- Video of QuickTime is going to play(happened twice)
- Only audio for QuickTime video is going to play(happened couple times)
- Only audio from video is going to be played for the length of QuickTime video (happens almost all the time)

Reproduced with:
5.5.0a6, 5.5.1p3, 5.6.0a1, 5.6.0b7

Note: I wasn't able to check regression because application doesn't work properly from <5.5.0a5(shows white screen)

DUT:
Samsung SM G930F, OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:samsung/heroltexx/herolte:7.0/NRD90M/G930FXXU1ZPK3:user/release-keys

Samsung SM G925F (Galaxy S6 Edge), OS:6.0.1, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zeroltexx/zerolte:6.0.1/MMB29K/G925FXXS4DPJ2:user/release-keys

Meizu MX5, OS:5.0.1, CPU:arm64-v8a, GPU:PowerVR Rogue G6200, Build:Meizu/meizu_mx5/mx5:5.0.1/LRX22C/m85.Flyme_OS_4.5.4.1.20151010121404:user/release-keys

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Fixed in: 2018.1.0b12, 2018.2.0a2