Translation:A mouse is smiling at me, so I'm laughing.
There's absolutely no difference in pronunciation here. Context will tell you which it is - a mouse or the act of smiling. It may help to remember that one is a noun (the subject of the sentence) and one is a verb (so it follows the noun/subject). Therefore, if you hear 'souris' as the subject noun (or an object noun), you know it's a mouse and the proper spelling. If you hear 'souris/sourit' as a verb (I smile/you smile/he/she smiles), you again know which it is by context. HTH. :-)