P1 audio static that is not present on Photon

Hello internet friends.

We’ve got a strange issue where our audio is just fine when playing from a Photon but experiences static on lower notes when playing from the same pin (A4) off the p1. This is a replicated issue on other similar pins from the p1. See video here for static example: https://vimeo.com/154651215 (worst at :09)

Is there any low-level or other reason why this would be occurring? This issue is replicated on two separate boards and different pins which seems to rule out any mounting or board design issues.

We are just calling using tone() and a basic pitches library via:

void iterateSounds(){

//Check if it's time to end the current note and play the next one
if (millis() >= currentSoundDuration + lastSoundTick){
    currentFromPitch = 0;
    currentToPitch = 0;
    currentSoundDuration = 0;

    //start playing the next note if there is one.
    if (!SoundQueue.isEmpty()){
        lastSoundTick = millis();
        SoundUnit note = SoundQueue.pop();
        currentSoundDuration = note.duration;
        currentFromPitch = note.fromPitch;
        currentToPitch = note.pitch;

        //play the note if it's not a rest (not 0)
        if(note.pitch > 0)
            tone(SPEAKER_PIN, currentToPitch);

//Check if note is a whistle that needs to be iterated
if (currentFromPitch > 0)
    iterateToneSlide(currentFromPitch, currentToPitch, currentSoundDuration);


void iterateToneSlide(int fromPitch, int toPitch, int duration){
    unsigned long progress = (millis() - lastSoundTick);
    int newTone = map(progress, 0, duration, fromPitch, toPitch);
    tone(SPEAKER_PIN, newTone);