Copy audio files to iPhone without using iTunes?
Yes, it is possible however Apple seems to have gone to great lengths to try to prevent you from doing this. Especially if you meant without the use of any sort of app on the PC at all, as if it were simply a disk drive in explorer.

That, I believe, is not possilble/practical without first jailbreaking your phone. If you do not have iOS 8 and you have not jailbroken your phone, I would suggest that you do while you still are able to and while it is a simple "one-button" process. While Pangu recently released a jailbreak for iOS 8, it is reportedly still a bit buggy so I would wait a bit if you have iOS 8. You would jailbreak your iPhone by downloading the pangu app on your PC from http://en.pangu.io/

However, if you don't want to jailbreak but you don't want to use iTunes, there are a number of different "replacements" for iTunes. Many people even go so far as to completely remove iTunes from their system as well.

Media Monkey is a rather full-featured replacement for iTunes that can do a number of things that iTunes can or will not. I personally liked MM when I was using it a year or two ago. They support Android as well.  You can download it at http://www.mediamonkey.com/download/

Another option is called iFunbox. They recently revamped their software but luckily they deigned to keep the older version, calling it iFunbox classic. Here's a quote from their website about it."


  With iFunBox Classic included, you can still manage files on your iDevice with no jailbreak needed. Here's some basic features:
  
  • Safe ipa package installation and easy access to files and resources in app sandbox. Learn More
  • Use your device as portable USB disk with hi-speed transmission beyond 20MB/s on iPhone and iPad.
  • Import and export songs, videos, photos and ringtones with iFunBox quick toolbox.


You can find iFunbox at http://www.i-funbox.com/

Hope this helps.