My smeg had the same problem with the timer beeps being very faint.  All I did was switch the cooker off at the main plug socket for 15 mins and when I switched it back on and reset the time, the timer beeps worked as noisily as before.  I would leave it switched off for more than a couple of minutes so all the settings are cleared from memory (as I think some ovens have a 'back-up battery' which stores info for a short time).
                 Mich
                
 September 2007