Although I have never used you r cam I am sure that a good add on mic will improve the sound. What is your budget?
Something like the SENNHEISER MKE300D is a good starting point at £179.99 from
www.keene.co.uk
Do a bit of searching as I'm sure I've seen this a bit cheaper elsewhere.
or if you want something a bit more then the Sennheiser K6 system is excellent, but the price is a fair bit more than the above mic. Do a search on here for it as I have suggested the K6 system in the past, but can't find a link to the kit now as I'm at work. expect to pay £500+ for it though.
Mark.