I think I've figured this out, if you go into Source/Content/XML/Preferences.xml in the extracted folder the values for musicVolume and sfxVolume should both be set to 0.25 but it seems like the leading zeroes are dropping off for some people. If you just update this to 0.25 again (or might be best to use 0 or 1 to avoid it being changed again) it should work for you.