Like the Cassingle? Get it at National Audio! Posted on 01/18/2013 by JonathanQ CALL OR EMAIL TRICIA TO FIND OUT MORE ABOUT THE CASSINGLE BY NATIONAL AUDIO. The “Cassingle” was a popular format for new single-track releases. We now produce the O-Card sleeves in-house, and at a great price! Get a package deal with ferric tape for 30 minute playback time. Link to Cassette Imprint Templates Link to O-Card templates Link to Quote request form.