Sound effects are audio clips that play when actions occur in a game, usually on-screen, sometimes off-screen.
Most sound effects are character voices and sounds made by strikes and other impacts, though all manner of sound effects can be employed when appropriate.
Among certain considerations, the volume of sound effects is a tough judgment call for the sound director. It must have a good balance with other audio (i.e. background music), but still be audible enough. If it has too much presence, the game will be annoying. If it lacks good presence, the game may sound a bit lonely, and those players who look forward to enjoying the sound, or those require it for some of their timing cues, will miss out.
A good example of awkward sound balance is SvC Chaos, where the striking effects are too quiet. KOF 2003 improved upon this slightly, with mixed reactions. After that, NEO·GEO Battle Coliseum and KOF XI put SNK fighting game volume back to something generally acceptable.
Successful hits make sounds that make sense for the attack. Whiffs are so called for the sound they make through the “air”. Blocking sounds vary, from a light “tick” (hence “tick throw”), to maybe something bumpy-sounding. Preemptive blocks, being more active than normal blocks, will often have a brighter sound in addition to their graphical effect (e.g., the “reflector” in Akatsuki Blitzkampf sounds like the pinging of clashing metal).
Street Fighter III has one unusual quirk: when Julian blocks, he has a different “tick” sound (something like if you hit a spring).