iTunes sound quality depends on the playback method. Is it though your Mac's analogue output port? The optical digital out? Through Airtunes via Airport Express, through apple TV? Through Airport Express's digital or analogue ports? You'll need to be more specific as there are many variables here.
It's quite possible that iTunes 9.1.1 better utilizes your system resources resulting in less dropouts or better data stream integrity, but if other devices are employed they may mask or obscure sound quality gains, or they themselves may be responsible for the sound quality improvement via firmware update or better connection methods.
Even a nearby fluorescent fixture will negatively affect sound quality, probably more so than a software update.