TL;DR — On a laptop that ships Windows 11 but was downgraded to Windows 10, the built-in speakers can go completely silent while headphones and USB audio work perfectly. The cause is usually not "the hardware is unsupported." On my ThinkPad E14 Gen 7 the audio driver registers its Dolby APO (Audio Processing Object) COM classes only inside an INF section gated to Windows 11. On Windows 10 that section is skipped, so the audio engine tries to CoCreateInstance an APO that was never registered, fails with REGDB_E_CLASSNOTREG, and the entire speaker stream fails to build — no sound, empty Volume Mixer, "Failed to play test tone." Registering the five missing classes by hand brings the speakers back (clean stereo). Everything below is reproducible with PowerShell, and every claim is backed by an HRESULT or an event-log entry rather than a guess. The same machine's built-in microphone was dead too — a sibling APO bug with the opposite fix; there's a companion section on it near the end.
Tested on
Machine
Lenovo ThinkPad E14 Gen 7 (Intel), machine type 21SX (21SXS0N500)
OS
