Playback/render lagging terribly, sound not working.

I have never had this problem before. I’m working on a project with just short of 4000 frames, FPS of 30. It was playing fine two days ago but today when I returned to work on it some more, playback was lagging, and sound had cut out completely, so I tried rendering it- and that lagged as well. I just exported the first 50 frames of the project, and the export doesn’t seem to be lagging, but it’s going to be really tough for me to work on this if playback’s gonna lag like this.

Any pointers, suggestions, or help?

Keep in mind that all the drawings making up the layers and frames of your animation
need to be loaded into the graphic card’s memory. The same graphic card that may
already be powering the displays of two or more monitors and whatever other programs
and web pages are loaded.

It may have worked fine at one moment but eventually it will reach a point where what
you are loading into memory exceeds the limitations of the hardware. For this reasons
it is recommended to limit Harmony scenes to one shot. The shots are then typically
combined into a full movie using editing software.

I understand this- how can I get the animation to work again, with less (or no) lag on the timeline, and sound functionality? Why isn’t the sound working in the first place? In fact, I’m having sound problems on multiple project files. How can I fix this?

“I understand this- how can I get the animation to work again, with less (or no) lag on the timeline”

Split the project into shots. Use the library to make templates to do this.

" Why isn’t the sound working in the first place? In fact, I’m having sound problems on multiple project files. How can I fix this?"

You’ll need to provide more information.

O.S. version
Quicktime version

Also make sure not to have spaces or special characters in the folder path of the project, the folder
you’re importing sound from, the project name itself or folder path where the project is stored.
Only use a-z, 0-9, “_” and “-” to name things.