Thanks 3dids, we fixed this in the Master branch. Our fix is similar. It did not make it into 4.7, but it will be in 4.8:
void UMediaSoundWave::PostLoad()
{
Super::PostLoad();
if (!HasAnyFlags(RF_ClassDefaultObject) && !GIsBuildMachine)
{
InitializeTrack();
}
}
void UMediaSoundWave::HandleMediaPlayerMediaChanged()
{
InitializeTrack();
}
This will also fix the cooking of media sound waves in packaged builds.