I have completed a beeping circuit with a dual 555 timer, from the 555 out to speaker, I feed the base of a transistor NPN tip 31 and feed 12 volts directly into the speaker to the transistor then to ground. From the out of the off/on astable side of the 555 I feed a tip 31 to short the circuit on/off to the speaker to get the type of beep duration I want. As I turn up the speaker power with a 5k POT, the frequency of the beep speeds up as the speaker takes more power from the circuit. I have a 2000uf cap in the circuit. Not sure why the beep frequency is increasing and what component to use or where to place it to keep the beep frequency the stable no matter what speaker volum is.