Marius

I'm assuming you'd want this to be played live, in which case you actually have another possible alternative which would also allow you to have as many other instruments playing as you wish. If you want the harp to be heard in a live performance which is using speakers, then you could have a mic hooked up to it and control the volume levels.

Of course this is only possible if it's a live performance with speakers, otherwise I'm afraid you're stuck doing it in one of the ways the others mentioned.

Good luck!