This is a windows behaviour - you can either turn off the tray notifications in the plugin config or disable the windows sound for "Systembenachrichtigung" in "Sounds und Audiogeräte"
Well, I used the option "disable windows balloon-tips" in MP set-up. That´s the reason why I didn´t see any balloon tips, else I may have brought this together with the plopp sound myself.
So maybe the MP option should be extended to turn off the sound, too, if this is possible? (From a usability perspective)
Not plugin but Last.FM related is that the tags attached to music is very irrelyable. I thnk they should use preconfigured tags instead of tags entered by users. For this Pandora.com is much, much better. I love the personalized radio and the loved tracks though!