You can increase it slightly through the "secret menu"
- Switch the ignition to "on" (as long as there's power to the radio, it can be at any position)
- Switch the radio on (if it's on already, switch it off then on)
- Immediately press and hold the [SEL] button - keep depressed for around 8 seconds until you see the Service menu. It will show a Serial Number.
- Move through the menu options using the station search keys [<] and [>]
- When you see GAL, set to the highest level (6) by pressing button [6]. This is the speed sensitive volume setting, 1 being lowest and 6 being highest.
- Switch the radio off to exit the services menu
Other things: increase bluetooth volume (only available on the iPhone when the bluetooth call is in progress). This is different from the usual volume setting.