Beep circuit modifications.

Longer or shorter beep.
For a shorter beep, C1 could be reduced to 5 microFarad. For a longer beep, C1 could be increased to 22 microFarad.

 
"oi" beep.
First, increase C1 to 47 microFarad and move to new position. This will make the oscillator voltage drop while it is running - making the beep shift tone.
Next, to control the cut off point where the circuit switches off, place a 5v6 Zener in TR1 base circuit as shown.

 
Different tone.
Change component values as below (see diagram):

R (three of) = 10K
C (three of) = 6n8
R4 = 33K
R5 = 4k7
This will increase the audio frequency to just below 1KHz.