Fix NS_E_MIXER_INVALID_LINE (0XC00D0FA5) in Windows

The 30-Second Fix: Restart the Audio Service

This is the one that works half the time. When Windows Audio Endpoint Builder gets confused about which line belongs to which device, you get 0XC00D0FA5.

Press Win + R, type services.msc, hit Enter.
Find Windows Audio Endpoint Builder in the list. Right-click it and pick Restart.
Wait 10 seconds, then restart your audio app. Should be clean now.

Why this works: That service is the middleman that maps hardware lines to software mixer entries. When it gets an orphaned or stale line reference — say after plugging in a USB headset, then yanking it out while an app was still using it — it misses the disconnect and keeps pointing to a dead line. Restarting forces it to re-enumerate everything from scratch.

Still broken? Move to the next step.

The 5-Minute Fix: Delete the Corrupt Audio Device Entry

If the service restart didn't cut it, the problem is likely a bad driver entry stuck in the registry. Windows caches audio device configurations in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\MMDevices\Audio. A corrupt entry there will cause the mixer to see a line that doesn't actually exist.

Unplug any external audio devices — USB headsets, external mics, HDMI audio. Only leave your default speakers or headphones plugged in.
Open Device Manager (Win + X → Device Manager). Expand Sound, video and game controllers.
Right-click each audio device and select Uninstall device. Check the box that says Delete the driver software for this device. Do this for every entry, including Realtek, NVIDIA, AMD, and any virtual audio devices like VB-Cable or Voicemeeter.
Reboot. Windows will reinstall the generic drivers automatically.
Re-test your audio app.

What's actually happening here: The registry under MMDevices stores a GUID for every audio endpoint — each line on your mixer. If that GUID points to a device that's been unplugged, or if the device's driver changed but the old line reference stuck around, the mixer tries to open a line that's crossed out. Uninstalling the device from Device Manager triggers a cleanup of those orphaned registry entries. The reboot repopulates them cleanly.

If this still fails, you've got a deeper problem.

The 15+ Minute Fix: Purge the Registry and Reinstall Drivers

This is the nuclear option. Some apps — especially older ones or DAWs like Audacity, FL Studio, or certain VoIP tools — cache their own mixer line references. But the underlying registry corruption can be stubborn.

Open Regedit (Win + R, type regedit). Back up the key first: right-click HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\MMDevices\Audio and select Export. Save it somewhere safe.
Delete the entire Audio key. Yes, the whole thing. Windows will rebuild it on next boot.
Download the latest audio driver for your motherboard or laptop from the manufacturer's site — not Windows Update. For Realtek, use their official package from realtek.com. For NVIDIA or AMD HDMI audio, get the GPU driver from the GPU maker.
Run the driver installer as Administrator. During installation, choose Clean Install if available (NVIDIA has this option; Realtek usually doesn't, but you can manually check Delete previous driver files in the installer).
Reboot twice. The first boot rebuilds the registry tree, the second boot stabilizes the driver bindings.

Why step 3 matters: Windows Update drivers are often stripped down. They lack the full mixer line definitions that some apps expect. A full driver package from Realtek or the chipset vendor includes all the auxiliary lines — like stereo mix, microphone boost, and SPDIF — that the mixer table needs. Without them, any app trying to open line index 3 might hit a null pointer, and that's your 0XC00D0FA5.

Still getting the error after all this? Then it's likely the app itself. Some programs (I'm looking at you, Skype for Business and old Audacity versions) hardcode line indices instead of querying the system. In that case, your only fix is to update the app or switch to a different audio API — WASAPI instead of DirectSound, for example.

One more thing: if you're on a corporate laptop with Dell or HP audio software (MaxxAudio, Waves, DTS), those overlays can mess with mixer line assignments. Try disabling them entirely in the startup tab of Task Manager, then reboot. They're rarely necessary for basic audio output.