JD-08: I can hear sound when I play the keyboard, but I can’t hear the sequencer playback sound.

Check the LAYER [TONE A], [TONE B] buttons on the JD-08. If one or both of these buttons are blinking, their respective parts are muted (silenced).

See “Playing Patterns - MEMO” in the “JD-08 Reference Manual” for how to mute/unmute a part.