diff --git a/audio/miles.h b/audio/miles.h
index 16ef8850c47..fde91e6227c 100644
--- a/audio/miles.h
+++ b/audio/miles.h
@@ -71,9 +71,9 @@ namespace Audio {
 // Miles Audio actually used 0x4000, because they didn't shift the 2 bytes properly
 #define MILES_PITCHBENDER_DEFAULT 0x2000
 
-extern MidiDriver *MidiDriver_Miles_AdLib_create(const Common::String instrumentDataFilename, const Common::String instrumentDataFilenameOPL3, const byte *instrumentDataPtr = NULL, uint32 instrumentDataSize = 0);
+extern MidiDriver *MidiDriver_Miles_AdLib_create(const Common::String &instrumentDataFilename, const Common::String &instrumentDataFilenameOPL3, const byte *instrumentDataPtr = NULL, uint32 instrumentDataSize = 0);
 
-extern MidiDriver *MidiDriver_Miles_MT32_create(const Common::String instrumentDataFilename);
+extern MidiDriver *MidiDriver_Miles_MT32_create(const Common::String &instrumentDataFilename);
 
 extern void MidiDriver_Miles_MT32_processXMIDITimbreChunk(MidiDriver_BASE *driver, const byte *timbreListPtr, uint32 timbreListSize);
 
diff --git a/audio/miles_adlib.cpp b/audio/miles_adlib.cpp
index 90c2c4b3f51..a68c252f4b8 100644
--- a/audio/miles_adlib.cpp
+++ b/audio/miles_adlib.cpp
@@ -1060,7 +1060,7 @@ uint32 MidiDriver_Miles_AdLib::property(int prop, uint32 param) {
 	return 0;
 }
 
-MidiDriver *MidiDriver_Miles_AdLib_create(const Common::String instrumentDataFilename, const Common::String instrumentDataFilenameOPL3, const byte *instrumentRawDataPtr, uint32 instrumentRawDataSize) {
+MidiDriver *MidiDriver_Miles_AdLib_create(const Common::String &instrumentDataFilename, const Common::String &instrumentDataFilenameOPL3, const byte *instrumentRawDataPtr, uint32 instrumentRawDataSize) {
 	// Load adlib instrument data from file SAMPLE.AD (OPL3: SAMPLE.OPL)
 	Common::File *fileStream = NULL;
 	uint32        fileSize = 0;
diff --git a/audio/miles_mt32.cpp b/audio/miles_mt32.cpp
index e36ccf1c5d9..dff863f1192 100644
--- a/audio/miles_mt32.cpp
+++ b/audio/miles_mt32.cpp
@@ -796,7 +796,7 @@ void MidiDriver_Miles_MT32::writeToSystemArea(byte index, byte value) {
 	MT32SysEx(targetAddress, sysExData);
 }
 
-MidiDriver *MidiDriver_Miles_MT32_create(const Common::String instrumentDataFilename) {
+MidiDriver *MidiDriver_Miles_MT32_create(const Common::String &instrumentDataFilename) {
 	MilesMT32InstrumentEntry *instrumentTablePtr = NULL;
 	uint16                    instrumentTableCount = 0;