someone can attempt to upload louder tones for you and make it a "theme" and add it into cydia for you but thats probably the only real option other than tonefx or sshing.
for those wondering how to change them make the sound you want a .caf file and name it "sms-recieved#.caf" the # sign being the tone your changing. for example sms-recieved2 will change the "tri-tone" ringtone.
Then create a folder called UISounds and put that file in it.
now ssh it into library\themes and open up winterboard and apply it.
that should make a "theme" that allows you to change only the sms tone
or to edit a theme just add the file to an existing winterboard theme
