Well, there have been some complaints about the speaker being one of the final secrets of the wii and I agree that technically it isn't a big thing but nevertheless I think it has potential.
For example.. imagine you are playing some sort of horror game (you remember this certain wii add with a guy and a girl using the controller like flashlights?).. the room is dark, perhaps you're playing in the late hours of the day.. and you're all alone in your living room. The game is kinda like project zero (which you should know if you like horror games) ... so you explore some eerie building or dungeon... the tension almost drives you mad as you know there most be *something* dangerous around here... launching itself at you at any moment.. and all of the sudden one of those japanese killer girl spirits starts whispering "I didn't want to kill..." and things like that.. and the sound is right next to you.. for it originates from your controller .. I think I would throw the thing as far away as I .. cursing the developers while at the same time thanking them for the good shock ^^.
Or another idea -> imagine you're playing a little girl in a dark eerie fantasy world (does anyone remember American Mc Gee's Alice?) and you have a teddy bear or something similiar as you companion and friend. The two of you have to get into that certain room over there, but theres that ugly guard... what to do??.. Distraction is the key =D you hide your teddy bear somewhere to lure the guard away from the door when you give him a signal.. this scenario would be even better if you assume there is some sort of dolby sound system connected to the wii... so you could put your remote some feet away from you, hear your teddy trying to get the attention of the guard and while you try to sneak in unseen you can hear the guard getting closer to your teddy/remote so you can estimate how long it will take him to come back...
I admit, these ideas might be a bit too complex but I hope you see that there are indeed some funny ways to incorporate the remotes speaker into gameplay. C&C welcome =)