add PCMStream:create_sound method

src/audio/MemoryPCMStream.cpp

@@ -61,3 +61,7 @@ bool MemoryPCMStream::isSeekable() const {
 }
 
 void MemoryPCMStream::seek(size_t position) {}
+
+size_t MemoryPCMStream::available() const {
+    return buffer.size();
+}

src/audio/MemoryPCMStream.hpp

@@ -31,6 +31,8 @@ namespace audio {
         bool isSeekable() const override;
 
         void seek(size_t position) override;
+
+        size_t available() const;
     private:
         uint sampleRate;
         uint channels;