Although im not aware of any problems with this card, I would however assume that if it works OK on Win98 then it should be OK on WInME as this is just a different flavor of Win9x. I would however suggest looking on monster sounds website or search Microsofts knowledge base for more information.
You will get no support or new drivers from Diamond for the MX300 because they abandoned that card a year ago. They told me it was incompatible wih my SE6 motherboard last summer. But I downloaded the latest Aural Win98 drivers and finally got it working.