Strictly speaking, you cannot make an ISO from an audio CD. Yes, you can make a bin cue file pair as another commenter has suggested. But realistically what you’ve then got is uncompressed wav audio with the metadata in separate files. The only real advantage this gives you is something that theoretically allows you to recreate precisely the original layout of the audio CD, together with the appropriate length of silence in between the tracks, etc.

When you convert to FLAC there is no loss in audio quality, you use approximately half of the storage space compared to wav, and you can have all of the metadata such as tags and art images embedded in the file itself.

Bin/cue is not really very useful unless you’re not listening directly from a computer or burning to a CD and listening to that. For every other use case, it’s better to have a file that you can play directly and index directly.