Are these among the type of files that are used to spread viruses/trojan horses?
AAC stands for "Advanced Audio Coding," and is Apple's preferred compressed-audio format. Think of it as "MP3: The Next Generation."
It was developed by the MPEG group. MP3's compression technology is over a decade old, and AAC is the follow-on codec using modern perceptual audio compression techniques.
The files are data, so they're not capable of spreading viruses or Trojans.