AudioCommon: set volume _after_ starting stream

Fixes an error with the CoreAudio backend, which apparently doesn't
allow you to set the volume before starting the stream:

```
59:31:087 AudioCommon/CoreAudioSoundStream.cpp:97 E[Audio]: error setting volume
```

This shouldn't cause any problems with other backends, since the mixer
starts with silence anyways.

Source/Core/AudioCommon/AudioCommon.cpp

@@ -57,8 +57,6 @@ void InitSoundStream()
     g_sound_stream = std::make_unique<NullSound>();
   }
 
-  UpdateSoundStream();
-
   if (!g_sound_stream->Start())
   {
     ERROR_LOG(AUDIO, "Could not start backend %s, using %s instead", backend.c_str(),
@@ -68,6 +66,8 @@ void InitSoundStream()
     g_sound_stream->Start();
   }
 
+  UpdateSoundStream();
+
   if (SConfig::GetInstance().m_DumpAudio && !s_audio_dump_start)
     StartAudioDump();
 }