The in-app dialer includes the ability to adjust audio settings.
Simply click the settings cog.
After clicking on the cog you will see mic and speaker devices available on your computer and may select your preferred device.
You can also test your speakers to ensure everything your selected device is connected properly for dialing by clicking the option to Play Test Sound.
Note: If call passthrough is enabled, the audio setting are unavailable, because the audio for the call is taken on by the device you are using for call passthrough.