(keitai-l) sound file types

Given the level of contributions on 
keitai-l, I realize I'm out of my league 
here but....

Which common sound-file types on
the internet (.wav, .mpg, . au, etc.) 
are playable on most (all?) cell-phones 
here in Japan? The reason: I'd like to 
embed links to very small (<10k) sound 
files into web-pages that are accessible 
to both computer browsers and cell phones.

Thank you.

--
Nicolas Cueto