fix(app): low vocal audio volume

app.py

@@ -116,7 +116,7 @@ def combine_vocal_and_inst(audio_data):
         wave_file.setsampwidth(2)
         wave_file.setframerate(audio_data[0])
         wave_file.writeframes(audio_data[1].tobytes())
-    command =  f'ffmpeg -y -i {inst_path} -i {vocal_path} -filter_complex amix=inputs=2:duration=longest -b:a 320k -c:a libmp3lame {output_path}'
+    command =  f'ffmpeg -y -i {inst_path} -i {vocal_path} -filter_complex [1:a]volume=6dB[v];[0:a][v]amix=inputs=2:duration=longest -b:a 320k -c:a libmp3lame {output_path}'
     result = subprocess.run(command.split(), stdout=subprocess.PIPE)
     return output_path