So I guess you are connecting external monitor (with build in speakers) with hdmi cable. In this case click on the apple logo in the top left corner of the screen and click System Preferences. In the system preferences window, click sound and under the output tab, select the HDMI option. This should produce audio (and video). If there is still no audio, click on the search button in the top right corner of the screen and type Audio MIDI Setup. In the list of search results, click on the Audio MIDI Setup application. In the Audio MIDI Setup application, click on the HDMI option in the list of devices on the left. In the right-hand portion of the screen, ensure that Format is set to 48000 Hz and 2-ch 24bit. The settings take effect immediately - just close the MIDI setup application.