@Ingolifs's answer to the question What was the last message to Opportunity today (13 Feb '19)? quotes ArsTechnica's Opportunity did not answer NASA’s final call, and it’s now lost to us:
Late Tuesday night, scientists at NASA's Jet Propulsion Laboratory sent their final data uplink to the Opportunity rover on Mars. Over this connection, via the Deep Space Network, the American jazz singer Billie Holiday crooned "I'll Be Seeing You," a song that closes with the lines...
To me, "Billie Holiday crooned..." suggests that a real audio recording of music was used to modulate the carrier in some way either direct AM or FM, or as digital data via more standard DSN data encoding schemes.
Is there any information available about how it was modulated or encoded?