To change your Skype for Business audio settings, open the client and click the small cog icon on the right-hand side of the screen. Choose Audio Device from the list on the left-hand side and your options will appear. You can choose your audio device that Skype is using for calls at the top of the screen, then test the speaker, microphone and ringer to make sure there is no fault with the equipment you are using.
If you want to add a secondary ringing device, for example, if you want your PC tower to ring when you have an incoming call, check the Also ring option and choose Speakers (High Definition Audio Device) from the dropdown list. When checking your microphone settings, be sure to check that your in-line headset controls aren't muted, or your speakers on Windows aren't muted in the bottom right hand corner of your screen.
If you continue to have issues, please raise a ticket and one of the MIS team will give you a hand.