tl;dr a single bit in the wobbles of the spiral used for recording.
From a technical standpoint, all CD-Rs (audio-specific or not) are formatted in a way that allows them to be used for audio and meet the Red Book standard that applies to audio CD recordings1. The distinction arises from licensing and royalties.
Each CD-R contains a permanent spiral pattern called the pregroove, formed at the factory. This spiral guides the recorder's laser during recording, and also includes a "wobble" at a 22.05 kHz carrier +/- 1 kHz FM, which encodes data known as Absolute Time in Pregroove (ATIP); ECMA-394 is the normative reference for this and other aspects of CD-Rs.
This ATIP data includes timing codes indicating the position on disk, but the portion of ATIP in the lead-in area also includes metadata. The key to your question is a flag that indicates either "Restricted Use" or "Unrestricted Use", stored in a special ATIP frame in one bit of the field that usually stores the number of seconds in the timecode. Quoting from ECMA-394:
Discs for restricted use are meant to be used in Professional CD-recorders only.
Within this category the General Purpose discs can be used for any application.
Special Purpose discs, carrying a registered Identification Code, are meant for that
registered application only.
Registration of the Identification Codes can be requested from Royal Philips Electronics
(for detailed address information see: “Conditions of publication” in this document).
Discs for unrestricted use may be used in Consumer CD-recorders and in Professional
CD-recorders
If so inclined, you could write a program that uses a computer's CD drive to read the ATIP using this IOCTL or its Linux equivalent, and you may be able to verify this difference for yourself.
Wikipedia confirms the market separation:
In the United States, there is a market separation between "music" CD-Rs and "data" CD-Rs, the former being notably more expensive than the latter due to industry copyright arrangements with the RIAA.[3]3 Specifically, the price of every music CD-R includes a mandatory royalty disbursed to RIAA members by the disc manufacturer; this grants the disc an "application flag" indicating that the royalty has been paid. Consumer standalone music recorders refuse to burn CD-Rs that are missing this flag. Professional CD recorders are not subject to this restriction and can record music to data discs. The two types of discs are functionally and physically identical save for this, and computer CD burners can record data and/or music to either.
The RIAA website confirms the rationale and royalty structure:
- It’s okay to copy music onto special Audio CD-R’s, mini-discs, and digital tapes (because royalties have been paid on them) – but not for commercial purposes.
- Beyond that, there’s no legal “right” to copy the copyrighted music on a CD onto a CD-R. However, burning a copy of CD onto a CD-R, or transferring a copy onto your computer hard drive or your portable music player, won’t usually raise concerns so long as [a few conditions regarding personal use and authorization are met]
Furthermore, an Amazon.com search does show that CD-Rs marketed for audio use seem to still be made and sold.
1 Supposedly, CD-RWs did (and may still) fail to meet the standard due to issues with optical reflectivity.
