Let me tell you a story about playing The Settlers IV on a 233Mhz 64MB RAM Voodo 3 2000. This game was cool in that it had two audio features: a soundtrack on the CD itself, played using the CD drive's DAC [^1], and you could also drop MP3 files into a folder where the game would play those instead. It was common practice to use a NoCD crack when playing online, because the CD check took long enough you could time out of the lobby, and if you forgot the CD you got booted. That meant most online gamers had MP3 files, and no CD.
The minimum requirements to play this game were a 200MHz CPU w/ MMX and 64MB RAM - I was pretty close to this baseline. So anyway, I discovered that the game played at much better FPS (like 30 instead of 5) if you turned the music off - but only when playing MP3 - CD Audio had no hit. Now perhaps the game used a sub-par audio codec, but that single MP3 decode stream was enough to make the game unplayable.
Anyway that's not to say that I would expect MP3 decoding to be a problem in 2014, in fact you can likely play audio with no noticable increase on CPU usage, but when you have multi-stream audio (think voices, background music, sound effects from various channels - guns, explosions, etc.) I can see it starting to add up - especially when the CPU is already constrained for the graphics, game logic and perhaps of course everyone's favourite anti-piracy/anti-cheat logic.
[^1] For younger readers, yes, CD Drives used to come with built-in DACs and a special cable you could hook directly into the audio card, allowing you to listen to CD Audio on PC for "basically" free in terms of CPU cycles.