Log into the DCS-5222L web UI from browser on a PC located on the same LAN as the camera. Navigate to SETUP > Audio and Video > AUDIO SETUP and make sure the Speaker and Microphone options are checked. Press the Save Settings button if you had to enable either of these two options.
Alternatively, the option to enable the speaker and microphone should be available from the mydlink.com website. After logging into your mydlink account, navigate to the DCS-5222L settings page to locate these options.