A notice can be sent to either a user or a channel. Where they are *displayed* is dependent on the options you set in your client.
As with the on text, you have two "parameters" after the "NOTICE" event type name: matchtext and target. In your code you only supplied matchtext, and left out the target.
So if you make your on notice look much the same as your on text, I think you will find it works as you want.
on *:NOTICE:*Message*:#: { /splay notice.wav }
(not sure if you were after the same matchtext or not - obviously replace with "*" to match any text. Ditto the target - use '?' instead of '#' for private notices, or '*' for both private and channel notices.)