There's something odd going on here- Its entirely possible to change the audio string, but not under the English language file- I think this is a bug on the JR end.
Short howto:
Open up the C:\Program Files\J River\Media Center 12\Data\Languages folder. Make a copy of the English folder- Rename this as you wish (The folder name becomes the text that shows up in the MC languages menu)
Now, open this new folder- There should now be a file named Language.txt - Open this with your preferred text editor. Now search for Audio- The first string that you find is the one you want to modify. Change the second line so that it now reads Music. Repeat this process for the Images string (Should be directly below the Audio string).
Slight thing to note- This'll alter the two labels at the top of the side bar as well.
Cheers
-Leezer-