My loop makes a little noise each time it starts

Hooo boy. Okay, this:

if(!this.s1)

    this.s1 = playSound("Loop1", {loop:-1});

else

    this.s1.play();

    this.s1.volume = 0;

This is a mess. The first part of the code will always execute, because this.sX is always never initialized. But if it ever was initialized, the else part would fail to set the volume, because you didn't put any curly braces around the entire else block. So it will only execute the first line of code after the else. And you have this code repeated six times.

Another problem is that you set every sound playing immediately, before any user interaction. So sound playback will fail on Chrome and any other browsers that only allow audio from a user interaction. All your code can be rewritten as this:

var onoff = [];

var sounds = [];

for (var i = 1; i < 7; i++) {

    this["btn" + i].id = i;

    this["btn" + i].addEventListener("click", volumen.bind(this));

    onoff = false;

}

function volumen(evt) {

    var n = evt.currentTarget.id;

    var on = onoff = !onoff;

    if (!sounds) {

        sounds = createjs.Sound.play("Loop" + n, {loop:-1});

    }

    sounds.volume = on ? 1 : 0;

    this["buho" + n ][on ? "play" : "gotoAndStop"](0);

}

Unfortunately this doesn't help with the looping delay. This appears to be an intrinsic limitation of CreateJS and/or HTML5 audio. I did notice that in IE the sound end event would sometimes fail to fire, causing looping to be delayed by a second or two. Importing the audio into Animate as a WAV file instead of an MP3 seemed to fix this. Importing MP3s directly into Animate does tend to cause problems.

Also, having your buttons in the exact shape of the birds is a really bad idea. The whole point of hit frames is to make the hit targets larger and without gaps so they're easier to click. Your bird button hit frame should just be a rectangle slightly larger than the bird, but not overlapping the other buttons.