What have you configured in "Hardware" -> "Sound" settings for your VM? When you run your VM, there is a toolbar in the upper right corner of the window title. In that toolbar, there is an icon of a speaker -- if you click on that icon a drop-down menu will appear. In that menu, check that the correct output is selected (an not the "DIsabled" option, then click on "Configure..." and ensure that in your VM settings you have "AC'97" as the Type in "Advanced Settings".
If all of this is configured correctly and you still have no sound, you need to check your VM's configuration (e.g. check `dmesg` for errors, check that ac97 modules are loaded, etc.).