Client is very quiet.

Firstly, no idea which category this should be under, as I'm not convinced its a bug or anything of the sort.

Anyway, my client hasn't been making any 'pings' or little 'dings' whenever I get an invite or the like. I checked out the settings and my volume mixer, but I couldn't find anything. I've missed a tonne of messages and invites to games because the client doesn't let me know. I might just be a massive numpty at the moment, but does anyone know why? Is there a button I've lost? Some setting I've accidentally muddled with? Help appreciated.