No problem, glad to help.

As for your current problem, try to store the original frequency in e.g. a global variable, than put an else after if(mouse_left) to restore the original frequency. So something like this:

var original_freq = whateverNumberYouWantHere;
....
....
if(mouse_left) //left mouse button pressed?
{
speed+=1; //forward speed
freq = 200*speed;
}
else freq = original_freq; //left mouse button NOT pressed
....
....

Could be I misinterpreted your question.