SoundStream: Internally construct the mixer

Instead of creating the mixer externally and then passing it in, it can just be made within the class.

Source/Core/AudioCommon/XAudio2_7Stream.cpp

@@ -143,9 +143,8 @@ bool XAudio2_7::InitLibrary()
 	return m_xaudio2_dll != nullptr;
 }
 
-XAudio2_7::XAudio2_7(CMixer *mixer)
-	: SoundStream(mixer)
-	, m_mastering_voice(nullptr)
+XAudio2_7::XAudio2_7()
+	: m_mastering_voice(nullptr)
 	, m_volume(1.0f)
 	, m_cleanup_com(SUCCEEDED(CoInitializeEx(nullptr, COINIT_MULTITHREADED)))
 {
@@ -185,7 +184,7 @@ bool XAudio2_7::Start()
 	m_mastering_voice->SetVolume(m_volume);
 
 	m_voice_context = std::unique_ptr<StreamingVoiceContext2_7>
-		(new StreamingVoiceContext2_7(m_xaudio2.get(), m_mixer, m_sound_sync_event));
+		(new StreamingVoiceContext2_7(m_xaudio2.get(), m_mixer.get(), m_sound_sync_event));
 
 	return true;
 }