diff --git a/src/sfizz/AudioSpan.h b/src/sfizz/AudioSpan.h
index 0791eafa..fa879312 100644
--- a/src/sfizz/AudioSpan.h
+++ b/src/sfizz/AudioSpan.h
@@ -200,7 +200,7 @@ public:
      * @param channelIndex the channel
      * @return Type* the raw pointer to the channel
      */
-    Type* getChannel(size_t channelIndex)
+    Type* getChannel(size_t channelIndex) const
     {
         ASSERT(channelIndex < numChannels);
         if (channelIndex < numChannels)
@@ -231,7 +231,7 @@ public:
      * @param channelIndex the channel
      * @return absl::Span<Type>
      */
-    absl::Span<Type> getSpan(size_t channelIndex)
+    absl::Span<Type> getSpan(size_t channelIndex) const
     {
         ASSERT(channelIndex < numChannels);
         if (channelIndex < numChannels)
@@ -402,7 +402,7 @@ public:
      *
      * @param length the number of elements to take on each channel
      */
-    AudioSpan<Type> first(size_type length)
+    AudioSpan<Type> first(size_type length) const
     {
         ASSERT(length <= numFrames);
         return { spans, numChannels, 0, length };
@@ -413,7 +413,7 @@ public:
      *
      * @param length the number of elements to take on each channel
      */
-    AudioSpan<Type> last(size_type length)
+    AudioSpan<Type> last(size_type length) const
     {
         ASSERT(length <= numFrames);
         return { spans, numChannels, numFrames - length, length };
@@ -427,7 +427,7 @@ public:
      *
      * @param length the number of elements to take on each channel
      */
-    AudioSpan<Type> subspan(size_type offset, size_type length)
+    AudioSpan<Type> subspan(size_type offset, size_type length) const
     {
         ASSERT(length + offset <= numFrames);
         return { spans, numChannels, offset, length };
@@ -440,7 +440,7 @@ public:
      *
      * @param length the number of elements to take on each channel
      */
-    AudioSpan<Type> subspan(size_type offset)
+    AudioSpan<Type> subspan(size_type offset) const
     {
         ASSERT(offset <= numFrames);
         return { spans, numChannels, offset, numFrames - offset };
diff --git a/src/sfizz/Voice.cpp b/src/sfizz/Voice.cpp
index 1c36df54..4a0fc5b0 100644
--- a/src/sfizz/Voice.cpp
+++ b/src/sfizz/Voice.cpp
@@ -615,7 +615,7 @@ void sfz::Voice::fillWithData(AudioSpan<float> buffer) noexcept
 
 template <sfz::InterpolatorModel M>
 void sfz::Voice::fillInterpolated(
-    const sfz::AudioSpan<const float>& source, sfz::AudioSpan<float>& dest,
+    const sfz::AudioSpan<const float>& source, const sfz::AudioSpan<float>& dest,
     absl::Span<const int> indices, absl::Span<const float> coeffs)
 {
     auto ind = indices.data();
diff --git a/src/sfizz/Voice.h b/src/sfizz/Voice.h
index 419c6584..9d5bfceb 100644
--- a/src/sfizz/Voice.h
+++ b/src/sfizz/Voice.h
@@ -376,7 +376,7 @@ private:
      */
     template <InterpolatorModel M>
     static void fillInterpolated(
-        const AudioSpan<const float>& source, AudioSpan<float>& dest,
+        const AudioSpan<const float>& source, const AudioSpan<float>& dest,
         absl::Span<const int> indices, absl::Span<const float> coeffs);
 
     /**