Creating millisecond accurate sequencer

(cross posting from Unity forum to see if you have any ideas)

I want to run a timer that says play a sound sample every 600 milliseconds. The accuracy is really important, as when played against a backing metronome track, any slight slip in time you can hear easily.

I've tried this:

float timeLastBeat;
Update(){

if(time.timeSinceLevelLoad - timeLastBeat >= 0.6f){
MasterAudio.play()
timeLastBeat = time.timeSinceLevelLoad;
}


i've also tried using Stopwatch


usingSystem.Diagnostics;
float timeLastBeat;

Start(){
timer = newStopwatch();
timer.start();
}
Update(){

if(timer.ElapsedMilliseconds - timeLastBeat >= 600.0f){
MasterAudio.play()
timeLastBeat = timer.ElapsedMilliseconds;
}


both create slight variations in gaps between beats, 600ms, 612ms, 650ms etc. I'm guessing this is an Update speed/frequency issue and even a coroutine would be the same.

I found this
docs.unity3d.com/ScriptReference/AudioSettings-dspTime.html

but I don't want to generate a sound from script, I want to play one using MasterAudio. I can't put a MasterAudio call inside OnAudioFilterRead.

Any thoughts?

Thanks