RF ceramic antenna inside aluminum case

I am making a project in which I will be streaming audio via RF from one device to another.

Due to the nature of the project, there cannot be a visable antenna. This leaves me really with one option - a ceramic antenna, mounted to the PCB.

My question is how much will this degrade my signal? The case has one opening for microphone on one side, but apart from that, the PCB will be completely enclosed.

An aluminum case will block RF. A plastic case will not.