1. Record your (stereo) audio track in your audio sequencer (like Logic, Cubase, whatever). Add a loud beep or loud peaky drum sample to the beginning of the track.
2. Mount the N9 to your computer. Then copy the track to N9. I have not tried wav files, if these don't get indexed, you may have to do a detour and encode the file to lossless flac format.
2. Go to Settings application -> Applications -> Music. Turn on the switch Dolby Headphone.
3. Connect your computer audio input to the headphone output of your N9 (assuming you have the cable)
4. Start recording on sequencer (another stereo track) and go to Music player on N9, locate your track and play back the sound.
5. Now cut the track so that it starts from the loud peak in the beginning.
6. Line up the original track and the track you just recorded visually - the peaks should align exactly with each other.
7. Now you can cut the synchronization sound from your track and mute the original track and you have a track with Dolby Headphone encoding on it.
This roundtrip will add noise of course because it goes through analog domain (DA->analog head phone amp->analog input->AD-conversion) and it will also otherwise reduce the quality slightly. However, many synthesizers I use have only analog outputs and it does not prevent me to use them and I have never had trouble with noise. The added noise may be negligible.
This is not the fastest way to do Dolby Headphone encoding for your audio track in your sequencer probably and certainly is not the official way, but this is a neat effect that you can do with the built-in software of your N9 and is kinda awesome feature. And I think the quality of the audio output in the N9 is so good that I could consider using this myself like I explained above.
I have been using this kind of syncing with a peaking signal in DSLR movie making (where I have recorded the sound separately with a audio recorder (I have Zoom H4N for that)) and it works because unlike in the prehistoric times when you could not sync two tracks if they were from separate machines (as there was speed variation, e.g. if you had two drum machines from same manufacturer, set both to 120 bpm and then record both separately and then try to line up the tracks, it started going out of sync in mere seconds if these were not midi synced with each other), but with modern equipment this is not a problem. The tracks will line up nicely.