In order to mark traffic over a specific wireless network, you would have to re-mark it before it leaves the radio if the application that originally sent the traffic did not mark it as voice.
So if you are using say FaceTime on wireless, if the application is marking traffic from the phone as Best Effort, it will leave the phone and reach the AP that way. If you want the responses back (downlink) to be prioritized, then you will need to specifically remark that application to Voice. The only way to get clients to send traffic as Voice is to get the application to mark them correctly.
I think you might find it difficult to do what your aiming to do, and in addition, if all the traffic on the wireless has EF/Voice marked, your going to run into issues with retries/contention most likely (depending on how many clients you have). This is because the Voice access category uses a contention window of 0-3 for the arbitration process. The more clients you have using that, the more chances you'll have of collisions etc., statistically anyhow.