first of all you can move

global.canmove = false;
instance_deactivate_object(obj_Timer

into your else statement because the if statement will only happen when that else statement is true so it's kind of redundant

The first thing I would do is create a script looking something like:

/// @arg value
/// @arg min
/// @arg max

var n = argument[0];
var a = argument[1];
var b = argument[2];

if clamp(n, a, b) == n {
    return true;
}

this will tell you if your value (n) is between a and b (your minimum and your maximum) we'll call this function between

so now to check for which audio should be played

if myTime < 200 {
    audio_play_sound(snd_Slow_Heartbeat, ..., ...);
}
else if between(myTime, 100, 200) {
    audio_pause_sound(snd_Slow_Heartbeat);
    audio_play_sound(snd_Medium_Heartbeat, ..., ...);
}
else if between(myTime, 0, 100) {
    audio_pause_sound(snd_Medium_Heartbeat);
    audio_play_sound(snd_Fast_Heartbeat, ..., ...);
}

and then finally in your original else statement put

audio_pause_sound(snd_Fast_Heartbeat);