Clive242's post animated me to have a closer look at my car to perhaps also increase the volume, but I didn't find any switch. I don't think that there actually is one in my case, because the speaker sits in a simple plastic housing, glued to the passenger side plastic cover in the trunk. It's not really visible at first sight or so, but having a crowded trunk with loose things flying around could actually damage the speaker. That's why I personally am rather disappointed by this cheap solution. Especially as it is the original Honda park distance control which had it's, not really cheap, price though.
Big
from my side.