This is a quite common issue.
Unfortunately, TomTom have never given us the individual volume controls that many of us have asked for.
In most cases the voice warnings will have been recorded as loud as the system will handle, but the software plays them back slightly quiet.
There are a couple of suggestions I can make which will help a bit
1. Use a "real" voice for the navigation instructions rather than a text to speech one.
The TTS voices have always been louder than the recorded voices, so you tend to have the overall volume set slightly lower which means the warnings are lower too.
The POI warnings are a better match in volume for the real nav. voices. Try it and see.
2. You don't say exactly what warning voices you are using (at a guess you are from the UK and maybe using the camera data and voices from PocketGPSWorld?)
If so, I had a lot to do with creating some of those voices and have spent a lot of time trying to get them as loud as they can possibly be.
So which voice are you actually using? It might not be the latest version or there may be some other oddity we can check.
There's also deeper discussion about it on PGPSW's own forum..