Yeah, you could install one on a 1m rubby ducky if you had space...!
I'm looking to install a sound system in my 5m rib - all you need is space for the actual radio -- you can either mount it into your console (get a waterproof fascia one), or get one that sits inside it's own fully waterproof housing, with drop down lid so you can access it.
And find somewhere to put it. Mine's going under my console as I have a big hatch there. So it's dryish within console, and then extra dry within it's dry case. (and I don't have space to flush mount it in the console...!)
Speaker wise, again all depends on your boat layout - I'm mounting one either side of my console as again I have a fair amount of room inside console box for them. But you might also be able to mount under seats etc...
Bear in mind, speakers will perform better if they are insulated from behind... i.e. build a speaker box around the speaker (with a drain hole) -- that way sound is insulated from behind and produces better output. Such a box will also protect the back of speakers from any bangs and knocks...
I did however struggle finding decent speakers for the wet environment of a rib --- most speakers advertised as 'marine' 'sprayproof' probably won't last that long on a very weather/elements exposed rib... Infact my local chandelery said they wouldn't recommend any of their speakers for a RIB!!! I did however find Aquatic AV producing some excellent, suitable speakers (http://aquaticav.com/
And yes, as 9d280 says, Im not even going to bother using the CD player... as soon as you go over anything other than a milk pond, you'll be skipping....
But the radio I'm looking at has SD card slot and USB flash drive slot.... perfect, as no moving parts on SD or Flash drives... and you can load up heaps of songs on say a 8gb flash drive...
You'll also be able to use it for FM radio with a suitable aerial of course... but of course reception will sink as you go to sea...
Re the arch speakers, yes you'll pay a shed load more for them than standard flush mount speakers, and you'll probably be getter lesser quality for your money. But they are great space savers... and can look quite cool....!
If you don't want to cut holes in stuff to mount flush mount speakers, you can get stuff like this: http://www.gme.net.au/products/speakers/marine/SPK009W
. (although don't know how marine proof that specific example is and they do look a bit dumb!
Hope that helps!!!