afconvert with find and exec.

Let's talk about your wildcard expansion. In your first attempt -

afconvert -d 'aac ' -f 'm4af' *.mp3 new1.m4v

The shell attempts to expand every file that ends in .mp3 into the arguments of the command. Something like-

afconvert -d 'aac ' -f 'm4af' file1.mp3 file2.mp3 file3.mp3 new1.m4v

According to the sparse manual and overly verbose help option the command accepts one source file and an optional outfile name.

In your second attempt, you are attempting to expand file names within find's exec function. This function is not a shell so wildcards are meaningless.

find . -name "*.mp3" -exec afconvert -d acc -f 'm4af' *.mp3 *.mv4 {} \;

The enclosing parenthesis "{}" contain the file name. You do not need to have a outfile name. afconvert will automatically name the file with the m4a extension. This should work- Of course, Use at Your Own Risk.

find . -name "*.mp3" -exec afconvert -d acc -f 'm4af' {} \;

Well, I'm guilty of using copy and paste then editing your find command. A list of audio file and data formats is available by issuing the command-

afconvert -hf