hi I have been to one this week aswell, this was one of the complaints I called for I have ordered a new timer unit (other stuff aswell)
It doesnt say in the instructions anything about changing the volume, its probably normal, my customer burnt her dinner because she didnt hear it!
the clock volume can be adjusted on some of the models, set the minute minder for 1 min, let it sound, while its sounding use the up/down set time arrows to adjust the volume, it normaly has 3 settings.