Playing the speech-only version.
20 decibels is a ratio of ten to one in amplitude, and sounds roughly four times quieter to a listener. It is much further down than most people expect: if you can comfortably hear the tune, the mix is almost certainly too loud to conform.
Measure it rather than guessing. In any audio editor, compare the loudness of a speech passage against a passage of background alone.
1.4.7 applies to prerecorded audio-only content that is primarily speech — a podcast, an interview, an audio guide. It does not apply to music, to singing, or to audio logos shorter than the threshold, and it does not apply to the soundtrack of a video (that is 1.2.x territory).
The simplest way to conform is route 1: do not put music under speech at all.