Polyphone SFZ -> SF2 Conversion
Although I see that some have posted about this free software already (http://polyphone.fr/), I wanted to clarify that this thing, besides creating soundfonts, can do a quick conversion from an .SFZ into an .SF2 file by simply opening an .SFZ and then saving as an .SF2. From what I've read, the main SFZ file that Musescore "officially" supports is the Salamander Grand. Apparently this has to do with the limited functionality of SFZ implementation within the Zerberus player. At any rate, what I'm practically saying is that if you have an SFZ that doesn't play well, maybe give this Polyphone program a go and convert your .SFZ to an .SF2 to see if it works in Musescore.