diff --git a/Outputs/Speaker/Implementation/CompoundSource.hpp b/Outputs/Speaker/Implementation/CompoundSource.hpp
index 6bc77220a..a4a9f2251 100644
--- a/Outputs/Speaker/Implementation/CompoundSource.hpp
+++ b/Outputs/Speaker/Implementation/CompoundSource.hpp
@@ -26,6 +26,20 @@ template <typename... S> constexpr bool is_stereo() {
 	return is_stereo;
 }
 
+/// @returns @c true if the variadic template arguments are ordered as all stereo sources followed by
+/// all mono; @c false otherwise.
+template <typename... S> constexpr bool are_properly_ordered() {
+	bool is_ordered = true;
+	bool is_stereo = true;
+	([&] {
+		if(S::is_stereo && !is_stereo) {
+			is_ordered = false;
+		}
+		is_stereo &= S::is_stereo;
+	}(), ...);
+	return is_ordered;
+}
+
 /*!
 	A CompoundSource adds together the sound generated by multiple individual SampleSources.
 	An owner may optionally assign relative volumes.
@@ -59,7 +73,7 @@ template <typename... T> class CompoundSource:
 
 				static constexpr bool is_stereo = S::is_stereo || CompoundSourceHolder<R...>::is_stereo;
 
-				// TODO: fix below for mixed mono/stereo sources.
+				// TODO: fix below for potential stereo -> mono conversion.
 				template <bool output_stereo>
 				void get_samples(std::size_t number_of_samples, typename SampleT<output_stereo>::type *target) {
 					// Get the rest of the output.
@@ -121,6 +135,9 @@ template <typename... T> class CompoundSource:
 		};
 
 	public:
+		// To ensure at most one mono to stereo conversion, require appropriate source ordering.
+		static_assert(are_properly_ordered<T...>(), "Sources should be listed with all stereo sources before all mono sources");
+
 		CompoundSource(T &... sources) : source_holder_(sources...) {
 			// Default: give all sources equal volume.
 			const auto volume = 1.0 / double(source_holder_.size());