That does sound strange.

There is a speed sensitive volume setting that raises/lowers the volume as speed goes up or down. The setting of that is done through the Bass/Treble, etc. button and the values it can be set at are 1-5. Usually 3 or 4 seems to work but I don't think there is any other adjustment for weather conditions (unless you have auto wipers and they have a feature I wasn't aware of).