I'm not really so sure it's missing one lead: it looks to me like it's hidden by a shadow from the bent-up edge of the capacitor.
But the cap is certainly, and seriously, damaged though. It's probably still working but with that kind of damage around the end cap seal it's safe to say it's days are numbered. When it leaks out it might fail-to-open in which case the audio circuit it's close to may simply stop working well on one channel. But it might fail-to-short which is the worst case if it's sitting across +12V as that would probably damage if not destroy your motherboard.
In summary, it's probably safest to take it to a good tech and have it replaced. Not too hard once the motherboard's back side is accessible which means removing it from the case. Short of that, remove the cap completely to avoid any possibility of shorting out +12V. Loss of on-board audio isn't such a big deal with a cheap add-in sound card, or even simply using USB headphones and mic.