Use Flirc GUI to record and pair remote buttons in Windows.

Controllers > Full Keyboard
Q = Volume Up
W = Volume Down
E = Mute

Move Flirc USB to Home Assistant and add this to configuration.yaml:

keyboard_remote:
  - device_name: 'flirc.tv flirc Keyboard'
    type: 'key_down'

In Home Assistant > Developer Tools > Events
Listen for events: keyboard_remote_command_received
Press the recorded keys to get the key_code.

Create automation:

alias: Remote - Flirc - Volume Up
description: ""
triggers:
  - event_type: keyboard_remote_command_received
    event_data:
      device_name: flirc.tv flirc Keyboard
      key_code: 16
    trigger: event
conditions: []
actions:
  - action: remote.send_command
    target:
      device_id: 2ba81e1706871d4857a3b97a7dbf749e
    data:
      device: Audio
      command: Volume Up
mode: single