I am outputting WAV files from Sibelius and uploading to web distribution channels such as Google Play and SoundCloud. I don't want the volume to be too quiet or too loud, but I'm not sure how to go about determining ideal levels.
Not really. There are too many other factors that come into play such as normalizing, limiting and EQ. If you set all your waves to the same max volume, they will still appear to have different volumes because of EQ, and dynamics, as well as several other factors.
There is no answer for this and so I can only recommend listening to your preferred artists on google play and soundcloud and try to match their volume (unless you start to distort, then you need to turn it down.)
A measured Dynamic Range value of between 6 and 10 will be adequate. Any louder than that, on renders with little performance expression, will end up with listeners turning it down.