diff --git a/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.cpp b/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.cpp
index 1377071359..d123e4cf2e 100644
--- a/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.cpp
+++ b/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.cpp
@@ -27,11 +27,6 @@ ResamplingAudioSource::ResamplingAudioSource (AudioSource* const inputSource,
                                               const bool deleteInputWhenDeleted,
                                               const int channels)
     : input (inputSource, deleteInputWhenDeleted),
-      ratio (1.0),
-      lastRatio (1.0),
-      bufferPos (0),
-      sampsInBuffer (0),
-      subSampleOffset (0),
       numChannels (channels)
 {
     jassert (input != nullptr);
@@ -67,6 +62,8 @@ void ResamplingAudioSource::prepareToPlay (int samplesPerBlockExpected, double s
 
 void ResamplingAudioSource::flushBuffers()
 {
+    const ScopedLock sl (callbackLock);
+
     buffer.clear();
     bufferPos = 0;
     sampsInBuffer = 0;
@@ -82,10 +79,12 @@ void ResamplingAudioSource::releaseResources()
 
 void ResamplingAudioSource::getNextAudioBlock (const AudioSourceChannelInfo& info)
 {
+    const ScopedLock sl (callbackLock);
+
     double localRatio;
 
     {
-        const SpinLock::ScopedLockType sl (ratioLock);
+        const SpinLock::ScopedLockType ratioSl (ratioLock);
         localRatio = ratio;
     }
 
diff --git a/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.h b/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.h
index e33396b893..49b4974b29 100644
--- a/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.h
+++ b/modules/juce_audio_basics/sources/juce_ResamplingAudioSource.h
@@ -76,12 +76,13 @@ public:
 private:
     //==============================================================================
     OptionalScopedPointer<AudioSource> input;
-    double ratio, lastRatio;
+    double ratio = 1.0, lastRatio = 1.0;
     AudioBuffer<float> buffer;
-    int bufferPos, sampsInBuffer;
-    double subSampleOffset;
+    int bufferPos = 0, sampsInBuffer = 0;
+    double subSampleOffset = 0.0;
     double coefficients[6];
     SpinLock ratioLock;
+    CriticalSection callbackLock;
     const int numChannels;
     HeapBlock<float*> destBuffers;
     HeapBlock<const float*> srcBuffers;
diff --git a/modules/juce_audio_devices/sources/juce_AudioTransportSource.cpp b/modules/juce_audio_devices/sources/juce_AudioTransportSource.cpp
index 34e05748c7..57c4c87c29 100644
--- a/modules/juce_audio_devices/sources/juce_AudioTransportSource.cpp
+++ b/modules/juce_audio_devices/sources/juce_AudioTransportSource.cpp
@@ -125,10 +125,7 @@ void AudioTransportSource::stop()
 {
     if (playing)
     {
-        {
-            const ScopedLock sl (callbackLock);
-            playing = false;
-        }
+        playing = false;
 
         int n = 500;
         while (--n >= 0 && ! stopped)
diff --git a/modules/juce_audio_devices/sources/juce_AudioTransportSource.h b/modules/juce_audio_devices/sources/juce_AudioTransportSource.h
index 5e4c21237f..67c4869907 100644
--- a/modules/juce_audio_devices/sources/juce_AudioTransportSource.h
+++ b/modules/juce_audio_devices/sources/juce_AudioTransportSource.h
@@ -167,7 +167,7 @@ private:
 
     CriticalSection callbackLock;
     float gain = 1.0f, lastGain = 1.0f;
-    bool playing = false, stopped = true;
+    std::atomic<bool> playing { false }, stopped { true };
     double sampleRate = 44100.0, sourceSampleRate = 0;
     int blockSize = 128, readAheadBufferSize = 0;
     bool isPrepared = false, inputStreamEOF = false;
diff --git a/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp b/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
index dd9344bcf8..6d289d114e 100644
--- a/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
+++ b/modules/juce_audio_utils/gui/juce_AudioThumbnail.cpp
@@ -214,7 +214,7 @@ private:
     std::unique_ptr<InputSource> source;
     std::unique_ptr<AudioFormatReader> reader;
     CriticalSection readerLock;
-    uint32 lastReaderUseTime = 0;
+    std::atomic<uint32> lastReaderUseTime { 0 };
 
     void createReader()
     {
@@ -645,6 +645,7 @@ bool AudioThumbnail::setDataSource (LevelDataSource* newSource)
     JUCE_ASSERT_MESSAGE_MANAGER_IS_LOCKED
 
     numSamplesFinished = 0;
+    auto wasSuccessful = [&] { return sampleRate > 0 && totalSamples > 0; };
 
     if (cache.loadThumb (*this, newSource->hashCode) && isFullyLoaded())
     {
@@ -654,22 +655,22 @@ bool AudioThumbnail::setDataSource (LevelDataSource* newSource)
         source->sampleRate = sampleRate;
         source->numChannels = (unsigned int) numChannels;
         source->numSamplesFinished = numSamplesFinished;
+
+        return wasSuccessful();
     }
-    else
-    {
-        source.reset (newSource); // (make sure this isn't done before loadThumb is called)
 
-        const ScopedLock sl (lock);
-        source->initialise (numSamplesFinished);
+    source.reset (newSource);
 
-        totalSamples = source->lengthInSamples;
-        sampleRate = source->sampleRate;
-        numChannels = (int32) source->numChannels;
+    const ScopedLock sl (lock);
+    source->initialise (numSamplesFinished);
 
-        createChannels (1 + (int) (totalSamples / samplesPerThumbSample));
-    }
+    totalSamples = source->lengthInSamples;
+    sampleRate = source->sampleRate;
+    numChannels = (int32) source->numChannels;
+
+    createChannels (1 + (int) (totalSamples / samplesPerThumbSample));
 
-    return sampleRate > 0 && totalSamples > 0;
+    return wasSuccessful();
 }
 
 bool AudioThumbnail::setSource (InputSource* const newSource)