not sure about the way snd_tune is working, but i used it to achieve the similar effect - in a piano simulator. i took 15 real note sounds of one octave and then changed their pitch with snd_tune, without noticing difference in duration.
i suppose you can make a simple loopable sound of 440 Hz and play it in the loop, use snd_tune to transpose it and snd_stop to stop it (or snd_tune to lower the volume till silence)