Audio out of sync in exported video on Resolve 15

Hi, I've been working on this project and did lots of work to it only to find out when it renders the video audio does not match the video in a rendered video file on Resolve 15. On the timeline audio and video is in sync and matches each other perfectly. However when I render it audio is delayed and out of sync with video. Does anybody know what is wrong?

How do you monitor in Resolve (audio and video)?

You don't specify things like media specs, project specs, export specs. It's difficult to find a problem without such details.

How do you monitor in Resolve (audio and video)?

Just normally in the timeline tab. In the little player above the timeline. Or do you mean something else here?

Yes, hardware.
Where are the speakers connected, are you using a proper I/O device or just the computer screen, etc.

I'm just using my macbook pro to do this edit. I think I know now where the problem is. I initially started my project on 24 fps timeline. However my footage is 50 fps. And I later realized maybe I should have chosen 25 fps timeline and I changed it midproject. However although it seemed like the change took place when I rendered my 25 fps timeline my audio was off. When I changed the timeline back to 24 fps audio became in sync again. Now I have another question. Because 25 fps timeline would be better for my 50 fps footage how much difference in quality there would be if I stayed with 24 fps? I'm planning to do some slow motion too. And it is quite tricky to understand how to properly do slow motion in Resolve. There is change speed option where you can choose to slow footage down to 50 % which gives 12.5 fps on 25 fps timeline. However if I chose retime curve there it is not possible to slow the footage down to exactly 50 % even if I'm on 25 fps timeline and have 50 fps footage. It gives decimal points like 50.33 % or 49.95 %. So if I want to use retime curve for speed ramping I can't chose exact percentage that I need for better math and most smooth footage on both 24 and 25 fps timelines. So is there a difference and what is better? How to properly do slow motion in Resolve? To go with 25 fps for better math I would need to redo the entire project but would I gain anything? Thanks for your advice

First of all: Use the timeline fps that fit your project. Anything else ist asking for trouble. Whatever you put into that timeline will be adapted to run in realtime.
Slo-mo is easy too: Go to "Clip Attributes" and change the speed to 25 fps (in your case) and you'll get a clean 2:1 slo-mo.

First of all: Use the timeline fps that fit your project. Anything else ist asking for trouble. Whatever you put into that timeline will be adapted to run in realtime.
Slo-mo is easy too: Go to "Clip Attributes" and change the speed to 25 fps (in your case) and you'll get a clean 2:1 slo-mo.

You're right. I tried rendering again with 24 fps timeline with 48 % slown down footage in 'change speed' option and it did not turn out very nice.
But I opened up a new project with 25 fps timeline and as you suggested changed clip attributes from 50 fps to 25 fps and it gave me slow motion. Thanks for that I did not know how to do it properly. Is that change clip attributes option essentially the same as 'interpret footage' in Premiere? What about change speed and retime curve options in Resolve? Are these not for slow motion? And how do I properly do speed ramping when I want to increase speed at some portions of my clip but to slow down other portions of the same clip? Thanks.

Yes, that's like "Interpret Footage" in Premiere.

Ramping is another solution, but it'll force Resolve to 'invent' new frames and that doesn't always look good and syncing is difficult.

And how to achieve clean fast motion in Resolve?

By shooting with a very fast camera and slowing it down where needed
Seriously, any method that needs synthesized frames (Blend or Optical Flow) will show more or less artifacts depending on image content.

I have just only started learning filming and editing so I need every advice I can get, even in the simple things like slow motion etc. Thanks.
I did not get what you mean by fast camera though .
So there is no way to achieve clean fast motion in my case? With 50 fps footage that I've got.

Let's get the wording straight here to avoid misunderstandings:
Do you mean making your motion look faster, called time-lapse, or do you want it to look slower? That's slow motion, often abbreviated as slo-mo. Do you want it to change it's speed while playing, like you see a lot in fight scenes or music clips? That's called ramping.

Slo-mo done right in software is not such a simple thing. Cameras from BM can do it right in camera: you set the project frame rate (your 'projector') to your final delivery speed, let's say 25 fps. Then you choose a different shooting speed, for example 50 fps. Done. It'll play in Resolve as a 2:1 slo-mo. Red does the same, but can attain higher speeds with more expensive models.

But slo-mo in post is easy if your timeline is slower than the clip from the camera: just change the speed in the Clip Attributes to the timeline speed. Perfect quality, no increased rendering times. But depending on the maximum frame rate your camera can do, you may only get 2:1 or 2.5:1.

Time-lapse is easy too at whole number ratios. Let's say you put your footage with 24 fps into a 24 fps timeline. Now change the speed to 400%: every fourth frame will play, three are dropped. No penalty in rendering speed or quality. But any off-rate or more slo-mo than your camera can deliver and the software needs to 're-invent' frames which were never filmed. That's always a penalty in rendering speed and/or image quality, depending on the method (Nearest, Blend or Optical Flow). No software as of today can do that as well as a camera built for it, shooting real frames. If you need in-camera ramping, rent a high-end Red or even a camera built for high-speed only.

And audio with speed changes is a whole different subject, ask a specialist.