To be honest I’m it’s not real clear what you are asking. I have no idea what merging MIDI tracks into the audio tracks even means. Having MIDI data aligned with an audio track makes sense. Merging it into an audio track does not make sense. We have a communication problem. The only thing I can think of is that whatever instrument the MIDI data was driving is now recorded onto the audio track and that you don’t want the MIDI track anymore. If that is the case, you could move the MIDI track to a folder, or make it small and mute it, or delete it all together.
You can’t set an audio track in front of or behind a MIDI track or vice versa. You can show/hide tracks, or move them up/down the list to set the next to eachother.