Often (typically) the RSSI is specified as a target for voice and data applications, as opposed to SNR. Of course, however, SNR is a very important factor! The Validated Reference Design for Skype for Business suggests a minimum SNR of 25 dB (see page 35). This seems like a good suggestion to me.
Ekahau Pro has a profile for planning Voice + Data networks and has a baseline SNR of 20 dB. But most of the other voice related settings are 25 dB.
The AP is generally not a factor in this calculation, but rather the SNR will be a result of the transmit power from the AP - the received signal strength at the client as well as any other received noise/interference.