What error is it showing? Your project is private so we can't see and try to replicate the error ourselves.
Is your audio project super top secret or can you actually show it so we can help you?
Are your copper areas nets named the same? GND on top layer and GND on the bottom layer?
Via nets also named GND?
Are the errors about via diameter or hole size? That would be a Design rule error.
Connecting two layers are done with vias, so you have something simple on your hands or a bug.

There is this really helpful thing andyfierman is always putting in his posts:
https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a

@deskpro256 reply : First, thanks for your kind reply. My both copper layers are named GND, and I have also tried to connect them with vias. Bit there is a little cross on via, indicating an error.
And my project is not secret, so I can share it, bit how?

I opened one of my projects and placed a via. It also gives me an error. You can see what errors you have in left side menu tab > Design Manager > Design Errors
The errors are about the Via diameter and Drill diameter, because in the design rules it is set up with different values and the default ones are not within those margins. Either change the design rules or the via parameters. All the following vias will have the same parameters.

Also note that the Net name for my placed via is empty. So after you place it, it won't connect to anything, because there are no other components that have the same non existing net name. If you add GND in there, it will want to connect the layers.

Also, if you don't want to manually place vias, there is a tool for that in the copper area. Select your copper areal layer, and in its attributes tab, on the bottom, there is a button "Place vias"

When pressed it opens a window where you can provide info about the via size and how often they will be placed in an array:

Also, making your projects public or private is simple:
Open your projects page> Hover over the project and there will be a "Manage" button, press that, it will take you to a new page:

Select "Settings" and there you can set it to private or public. Save the changes and you're done.
Hope it helps!

@deskpro256 reply: Thank you very much for your help. I have given a try and it worked flawlessly. Once again thank you for your kind assistance.

I realize this is a year old thread... but my question is a piggyback, so its very relevant, I think ;)

I have the same problem, except that my two copper areas are on the same layer. But with each other's clearance settings, I obviously cannot overlap them... so how do I connect them?  I tried dropping a third copper area across the two with 0.00 clearance, but that didn't seem to close the gap between them either.

this is my very first time designing a pcb, so please pardon my ignorance - I appreciate any advice or solution!
Chris

I think I answered my own question.... I was not showing the ground "wire" connection between the two voltage levels, so it was not on the PCB, and hence I could not "connect" the two.  I added this wire to the schematic, and donzo - one copper area does it! ;)