thanks for your answer. The properties for the music file:
File format: Microsoft WAV Files
Attributes: 44,100kHz; 16 Bit; Stereo
The audio files are otherwise the same, apart from the sample rate which is 48,000kHz. Furthermore, the file information (Windows, hovering the cursor on top of a file) says that the music file's audio quality is 'CD' (Lossless) and for the audio files it's 'DAT' (Lossless). Don't know if that's a possible reason for this - but then again I tried to mix the music file with one of the audio files using the good old Windows Sound Recorder, and the result played just as well as an individual file - no crackling.
If you could test the files, I would really appreciate it.
These probably open in Real / Media Player once you click the link, but you can then right click -> save as.http://users.metropolia.fi/~m0702383/Music.wav http://users.metropolia.fi/~m0702383/Audio.wav