If you are using the OGG file format, as used by the latest TT systems, for additional warnings then the easiest way is to use something like Audacity.
See http://tomtom.gps-data-team.com/tomtom_voices.php and tweek up the volume on the ogg file itself.
Thanks Mate...sorry for the delay in acknowledging your response.
that helped Ok, but when I record my own voice with audacity (in ogg format) my TomTom does not recognise it. I understand from reading other posts in the forums that TomTom doesn't necessarily recognise these alerts, as other people seem to have a similar problem with activating .ogg alerts.