Push_sound doesn't work properly, so just set it to none and target the button to a target_speaker.
Using the noise key on this entity will make the sound be played every time the button moves, which means the sound will be played twice when someone pushes the button. Once when it goes in, and the other when it returns to its original position.
Haven't tried using the key pos1 but I can if there's anyone interested.
Thanks to vulture and NuLL
[12:25] <JohnnyEnglish> morning Nounou
[12:25] <JohnnyEnglish> wotcha doing?
[12:25] <Nounou> hello
[12:26] <Nounou> nothing much, i've nothing to do at work so
[12:26] <Nounou> modeling woman
[12:26] <JohnnyEnglish> woo
[12:26] <JohnnyEnglish> real women?
[12:26] <Nounou> realistic yes, on maya