Sample-precise sync on multiple audio sources ?

Hi guys, I am making a sort of augmented reality dj machine with unity+vuforia.

I basically have 4 instruments, one audio source per each, and swapping in different loops according to the markers shown in front of the camera. To work nicely they need to be synced as precisely as possible (all the loops are the same lenght, and only one loop per instrument can be played at the same time). I keep a silenced audiosource with one of the loops (so that it has the right lenght) looping endlessly as a reference, so that whenever a new loop is added it starts from the exact same point, therefore giving an illusion of "skill" for the user (i basically just sync each new loop for him).

I am going crazy because of the syncing exactly. I tried the most basic approaches based on time (newLoop.time = refTrack.time) then i moved to the timeSamples to make it more precise. Still something is wrong. (Latency is set to best latency and the files are uncompressed Wavs).

I use a function with ayield return new WaitForFixedUpdate(); called from a StartCoroutine when a new loop has to be played and synced, and I set the audioSourcesvelocityUpdateMode = AudioVelocityUpdateMode.Fixed; In my mind it looks like the most logical way to make sure it happens all at the "same time".

Right after the syncing I print the timeSample value of the ref track and the loop to sync and it looks all fine, they show the same timeSample stamp

 switch (track) { 
     case 0 : drum.Play();
     drum.timeSamples = referenceSync.timeSamples; 
     drum.volume = 1f; 
     Debug.Log("Ref "+referenceSync.timeSamples); 
     Debug.Log("Drum "+drum.timeSamples);
     break;
  }

then, checking on a debug on screen, or even just on the inspector in a public variable, several times they are not. They tend to have an error offset of almost fixed values, that make me feel like it's frame based (470 or 940 usually, but sometimes i ahve some variantions too).

The same identical problem i have in the editor, but also in the Android build. Does anybody have any idea on what is the cause for such a weird behavior and what am I doing wrong?

Thanks in advance, cheers, H