A frequent complaint from users on Trustpilot and support forums is receiving the error message "The product key you typed is already in use" . This typically happens if you attempt to activate the software on more computers than your license allows. In such cases, contacting Letasoft support is the only solution, as using an already-active key on a new machine is a clear violation of the license terms.
If you find the standard volume of your Windows apps too low, you can try these safer methods: letasoft sound booster product key github work