Use Juce CriticalSection for main audio processor too. Use tryEnter for changing main volume parameter. Looks like it's best to do the tryEnter stuff for everything possible...

Source/PS_Source/StretchSource.cpp

@@ -134,9 +134,12 @@ void StretchAudioSource::setMainVolume(double decibels)
 {
 	if (decibels == m_main_volume)
 		return;
-	ScopedLock locker(m_cs);
-	m_main_volume = jlimit(-144.0, 12.0, decibels);
-	++m_param_change_count;
+	if (m_cs.tryEnter())
+	{
+		m_main_volume = jlimit(-144.0, 12.0, decibels);
+		++m_param_change_count;
+		m_cs.exit();
+	}
 }
 
 void StretchAudioSource::setLoopXFadeLength(double lenseconds)

Source/PluginProcessor.cpp

@@ -226,7 +226,7 @@ void PaulstretchpluginAudioProcessor::startplay(Range<double> playrange, int num
 
 void PaulstretchpluginAudioProcessor::prepareToPlay(double sampleRate, int samplesPerBlock)
 {
-	std::lock_guard<std::mutex> locker(m_mutex);
+	ScopedLock locker(m_cs);
 	if (getNumOutputChannels() != m_cur_num_out_chans)
 		m_ready_to_play = false;
 	if (m_using_memory_buffer == true)
@@ -301,7 +301,7 @@ void copyAudioBufferWrappingPosition(const AudioBuffer<float>& src, AudioBuffer<
 
 void PaulstretchpluginAudioProcessor::processBlock (AudioSampleBuffer& buffer, MidiBuffer& midiMessages)
 {
-	std::lock_guard<std::mutex> locker(m_mutex);
+	ScopedLock locker(m_cs);
 	ScopedNoDenormals noDenormals;
     const int totalNumInputChannels  = getTotalNumInputChannels();
     const int totalNumOutputChannels = getTotalNumOutputChannels();
@@ -394,7 +394,7 @@ void PaulstretchpluginAudioProcessor::setStateInformation (const void* data, int
 	if (tree.isValid())
 	{
 		{
-			std::lock_guard<std::mutex> locker(m_mutex);
+			ScopedLock locker(m_cs);
 			for (int i = 0; i < getNumParameters(); ++i)
 			{
 				auto par = getFloatParameter(i);
@@ -416,7 +416,7 @@ void PaulstretchpluginAudioProcessor::setStateInformation (const void* data, int
 
 void PaulstretchpluginAudioProcessor::setRecordingEnabled(bool b)
 {
-	std::lock_guard<std::mutex> locker(m_mutex);
+	ScopedLock locker(m_cs);
 	int lenbufframes = getSampleRate()*m_max_reclen;
 	if (b == true)
 	{
@@ -462,7 +462,7 @@ String PaulstretchpluginAudioProcessor::setAudioFile(File f)
 			//MessageManager::callAsync([cb, file]() { cb("Too high bit depth in file " + file.getFullPathName()); });
 			return "Too high bit depth in file " + f.getFullPathName();
 		}
-		std::lock_guard<std::mutex> locker(m_mutex);
+		ScopedLock locker(m_cs);
 		m_stretch_source->setAudioFile(f);
 		m_current_file = f;
 		m_using_memory_buffer = false;

Source/PluginProcessor.h

@@ -104,7 +104,7 @@ private:
 	void finishRecording(int lenrecorded);
 	bool m_using_memory_buffer = true;
 	int m_cur_num_out_chans = 2;
-	std::mutex m_mutex;
+	CriticalSection m_cs;
 	File m_current_file;