Start xev to find the keycodes. You get as result somthing like:

KeyRelease event, serial 43, synthetic NO, window 0x2400001,
    root 0x3d, subw 0x0, time 3939566, (84,27), root:(1498,49),
    state 0x10, keycode 232 (keysym 0x0, NoSymbol), same_screen YES,
    XLookupString gives 0 bytes:

So we have keycode 232 for this key. Now edit .xmodmaprc in the following style:

keycode 232 = XF86AudioMedia

Allowed symbols can be found in /usr/X11R6/lib/X11/XKeysymDB.

My KeySonic has the following keys:

! config for KeySonic
keycode 153 = XF86AudioNext
keycode 162 = XF86AudioPlay
keycode 144 = XF86AudioPrev
keycode 164 = XF86AudioStop
keycode 176 = XF86AudioRaiseVolume
keycode 174 = XF86AudioLowerVolume
keycode 178 = XF86HomePage
keycode 236 = XF86Mail
keycode 234 = XF86Back
keycode 233 = XF86Forward
keycode 229 = XF86Search
keycode 231 = XF86Refresh
keycode 230 = XF86Favorites
keycode 232 = XF86AudioMedia
keycode 160 = XF86AudioMute
keycode 227 = XF86WakeUp
keycode 223 = XF86Sleep
keycode 222 = XF86PowerOff

Now load the settings with:

xmodmap .xmodmaprc

In Beep-Media-Player activate the plugin „XF86Audio Keys Control“ and everything should work.