Open Filebox on your N900 , or use WinSCP on your computer to access root files on ur N900 using wifi network ..
then simply go to this folder :
/usr/share/sounds
this folder contain sounds of ur N900 like IM tone , SMS tone , ..etc >
some tones types are "WAV" like IM tone .. so you can just find another tone that must be ".wav" and change it's name with same name of the original tone in ur N900 , and then copy-and-replace .. it's done ..
>>>>>>>
If u r interesting , I will share u some of my tones. I changed them with other tone that I like ... u can just download my zip folder, unzip it and copy-and-replace the tones in ur phone "sounds folder" , and see if sounds changed or not .. or just restart your phone if nothing happened ..
>>>
this my favorite tones , it will replace the sounds of : new IM message when u out of chat window - new IM message when u in chat window - IM accounts log in - power off phone ..
they are collected from windows7-yahoo messenger- hotmail messenger .