See this wiki for projects that have existing timelines in resolve and you just want to add your surround mix and export

Davinci Resolve - How to Import Surround Sound Mix and Export Surround File

Open Davinci Resolve

When you get trough the prompts

Open a New Project or Existing Project

Click on Media Tab and add 5.1 audio clip to media pool

Find the clip with the 5.1 interleaved audio that was exported

Right click it and choose "Add into Media Pool"

Choose to Change the frame rate

Click "Change"

Next - Go to the Edit Tab

Right click in the Media Pool area

>Choose >Timeline>CreateNew Timeline

Set the Timeline to be 1 Video Track

1 audio track that is 5.1

Uncheck Empty Timeline

Click on "Create"

Now add you video on to the timeline with the surround sound audio file set to 5.1

Next go to >Fairlight>Bus Format

Bus 1

Choose Format 5.1

Click on OK

To Start the Self Contained QuickTime export of the Final Resolve Project click on the Deliver Tab

In the upper left corner click on Render Settings

Choose >ProRes>ProRes Master

Make Sure Single Clip is selected and

Title the Filenmae and choose a location to save the file

Click Browse to navigate to the folder you want to save the QuickTime

Pick a Storage Drive and folder to save the file

Click OK

Make sure the Export Video is checked

Choose Format=QuickTime
Codec Apple ProRes 422 HQ (use for RTF Screenings)

Or what ProRes Codec you'd rather use for lower or higher quality

1920 x 1080 HD (use for RTF Screenings)

Or if you would like to export a UHD 3840 x 2160 version choose that setting

FPS 23.976 (or 24 or whatever frame rate your timeline is)

Choose Advanced Settings

Make sure Retain Sub-black and Super White Data is checked

Scroll back up in the tool and choose the Audio Tab

Check the Export Audio box and set to Linear PCM, 24 Bit and Bus 1 stereo

Audio tab set to Bus1 5.1

Finally, if you are trying to playout via VLC OR ARE ADDING CAPTIONS you will need to change the timeline Start Timecode setting to start playback at 0 hour.

Go to DaVinci Resolve locate the sequence you want to change in the Bin

Right click it and >Timelines>Starting Timecode

Set TimeCode to 00:00:00:00

Choose to Start Render to create the QuickTIme

Once the file renders open in Quicktime and choose 

>Window>Show Movie Inspector

Should see audio settings as surround 5.1 (C, L, R, Ls, Rs, LFE)

Back in your timeline...

To export the generated subtitles as a .srt file, navigate to the subtitle track and right click

Click on Export Subtitle...

This menu should be called up

Name your file and make sure you're saving it to the right destination

Click Save