An alternative audio device to an AV receiver can be a mini system, sound bar, or the TV itself.  You can add activities for these devices because they're able to control the sound.

Examples of activities that involve volume control:

• Watch TV (see answer 360023250334)

• Watch a Movie (see answer 360023250334)

• Listen to Music (see answer 360023250334)

The Harmony software defaults to your TV for volume control unless you have an AV receiver, mini system, or sound bar device in your account. If you have multiple audio devices, you'll have to choose which audio device controls the volume for each activity. You can also create multiple activities to use all of your audio devices.