Fix DTK audio not working after loading a savestate

The main problem was that the volume of the mixer wasn't savestated.
The volume is typically 0 at the beginning of a game, so loading a
savestate at the beginning of a game would lead to silent DTK audio.

I also added savestating to StreamADPCM.cpp.

Source/Core/Core/HW/AudioInterface.cpp

@@ -127,6 +127,8 @@ void DoState(PointerWrap& p)
   p.Do(g_AISSampleRate);
   p.Do(g_AIDSampleRate);
   p.Do(g_CPUCyclesPerSample);
+
+  g_sound_stream->GetMixer()->DoState(p);
 }
 
 static void GenerateAudioInterrupt();