Need some amplifier sound quality help

Guys,
I've got this MOSFET amplifier in 3 different boxes. I've got this little tone playing a warble.
In 2 of the boxes it sounds great, like a cell phone ringing. In the third, it starts great but ends sounding like someone farted after the last note. I thought initially it was because I didn't have the series cap into the gate, so I just finished adding one - however the sound didn't change at all.
The only difference is in the size of the box - 2 are smaller, Staples Stretch pencil boxes. The farty one is a large 4L box. Both box types have holes drilled in front of the speaker.

Any ideas?
Thanks
Robert

// info on alarm sound
 #include "g_pitches.h"

// notes in the melody:
int thisNote = 0;
int noteDuration = 0;
int pauseBetweenNotes = 0;
int melody[] = {
NOTE_C6, NOTE_A5, NOTE_C6, NOTE_A5, NOTE_C6, NOTE_A5, NOTE_C6};
// note durations: 4 = quarter note, 8 = eighth note, etc.:
int noteDurations[] = {
12,12,12,12,12,12,4};
  // create a warble once
  for (thisNote = 0; thisNote < 8; thisNote++) 
  {
    // to calculate the note duration, take one second 
    // divided by the note type.
    //e.g. quarter note = 1000 / 4, eighth note = 1000/8, etc.
    noteDuration = 1000/noteDurations[thisNote];
    noTone(6);       //apparent known bug - need this for the tone to play next.
    tone(6, melody[thisNote],noteDuration);
    // to distinguish the notes, set a minimum time between them.
    // using the note's duration + 10%:
    pauseBetweenNotes = noteDuration * 1.10;
    delay(pauseBetweenNotes);
    // stop the tone playing:
    noTone(6); 
  }
  digitalWrite (speakerOut, LOW); // turn off the amp
  delay (500);

Hi Richard,
All the boxes are drilled out in front of the speakers, I just didn't have a picture showing that.
The LM386 might have been fine; I had the MOSFETs on hand and I wanted to use them. The LM386 is also only good for like 1W, I am getting 2.5-3W I think out of the speaker, and I'm sure its good for more but the 3W is plenty loud.
The sound is awesome except for the blurp at the end. I thought for sure the series cap would take care of it as I had built that into the smaller boxes after the large box was built.
I can try swapping the speakers around, they just unscrew and the speaker & cap have wires with connectors so will be easy to move from box to box.
Robert

I've a couple of observations.

Firstly the MOSFET is rated for 4.5V gate and above - you are feeding it a +/- 2.5V AC input which is borderline in driving it into conductiur on - I'd lose the blocking capacitor to the gate (even this seems not to be the issue here).

Secondly the MOSFET is a switching device, and a loudspeaker is an inductive load. You might be needing a free-wheeling diode across the 68 ohm resistor to tame any turn-off spikes - oscilloscope will show if this is an issue - in fact as the MOSFET is only rated at 30V there's not much headroom (although modern MOSFETs are rated for repetitive avalanche breakdown and the 2707Z is pretty beefy).

Turn off spikes might account for harsh sound too.

Ok, I'll put a scope on it. The sound is not harsh at all tho. More like someone blowing a raspberry after the warble.