When you say 'speaker' do you mean something which will play music? Or just something to give a warning sound (i.e. a buzzer)? The former would require additional music source/amplifier circuitry whereas a buzzer is self-contained and available off-the-shelf cheaply.

However is there another accurate way of setting my high and low voltage?

Use a programmable voltage reference, e.g. a TL431

 

Thanks Alec, I'll look into the programmable reference if the voltage dividers don't work out.