Game audio has obviously come a long way from the 8-bit days. While the bleeps and bloops that made up NES game soundtracks may be what the non-gaming public associates with video games, today games often have fully orchestrated scores played by live musicians. Sound effects, too, have become far more complex, varied, and interesting. How many different footstep noises does the last game you've played have in it? Probably a lot more than you'd expect.
While game audio has improved immensely over the past couple decades, it's also becoming much more difficult to do consistently, and far more potential problems crop up. I'd like to list a few that I've encountered over the past few years, but before I do, let me say that my experience with recording and mixing audio comes primarily from a musical background and not a game audio background. Nevertheless, I think these potential flaws are ones that can be recognised and corrected in good game audio production.
Voices too low in the mix
This is something that I notice almost immediately, perhaps because of my musical background. In almost every genre of music (excepting some heavy metal or experimental acts) vocals are mixed significantly louder than any other aspect of the song. This is partly because vocals are often the centre of the song, but also because, unlike other instruments, vocals don't usually blend in as well and need to be clear.
I've noticed in a lot of games lately, though, that the voices are mixed far too low. This is especially problematic in games with a heavy emphasis on story. Mass Effect is probably the worst offender in this regard - sometimes the ambient crowd noise is louder than the main characters speaking!
Main speaking parts should always be clearly audible above every other sound, including music, foreground sound effects, and background ambience. That means they need to be loud. Unless there is a specific reason for the voices to sound muffled or distant (being underwater, for instance), they should be at the top of the mix. The player should never have to strain to hear the dialogue.
The sliders in the option menu don't affect the sounds they should
This one usually compounds the previous example. If there is a sound effects slider, turning it down should affect all non-dialogue, non-music sounds in the game. It should not just effect the menu noises.
I've encountered games where I think I've adjusted the audio mix to a more reasonable level, only to discover that only some of the sound effects are quieter and the rest are still drowning out the speech, or are still rumbling my sub-woofer. Which leads to . . .
Your explosions are too loud
I'm sorry, I'm sure you're very proud of how realistic and impressive and big and booming your explosions sound, but they're too loud. My neighbours do not want to hear them. Game audio should be mixed for living rooms, not movie theatres.
I've played plenty of games where the sounds seem to be well mixed and well balanced until something blows up, and then suddenly I need to turn my speakers down. But only until the explosions are done, because the appropriate volume on my speakers for explosions and every other sound in the game is not the same.
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Game audio has come a long way, and is leagues better than it was even ten or twelve years ago. But while the quality of audio available to us has increased significantly, the mixing of that audio is often extremely uneven. A good game audio mix should make everything clear that needs to be clear, and keep all of the sounds at volumes relative to the rest of the mix.