hey i was wondewring maybe Format Factory. its a free multimedia converter with many more advance ways to change the settings of the conversion. the site is http://www.formatoz.com
even though it is not open source but i would think the author wouldnt mind if somebody made it portable with his/her permission.
FormatFactory Portable used to be developed here. However due to direction the author was taking with the addition of an ebay banner and AdSense registry key, it was discontinued.
There is currently available XMedia Converter Portable. I switched from FormatFactory to it when the change happened here and have had no complaints. Has done everything I used to do with FormatFactory.
Hope that helps
Also please remember to use the search function in future.
PortableApps.com Advocate