Hello Phillip_1997,
This is a function that's enabled by default to avoid any echo caused by recording said background music as you're recording your voiceover. This option can be changed by clicking the Voiceover Options button as detailed below.
To adjust the recording settings, do any of the following:
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Change the input device: Click the Voiceover Options button , and then click the Input Source pop-up menu and choose an option.
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Adjust the input level of the microphone: Click the Voiceover Options button, and then drag the Volume slider right to increase the volume of what is being recorded, or left to decrease it.
You can monitor the audio levels using the audio meter to the left of the red Record button. You should set the volume so that the audio meter stays green, even when your voice is at its loudest.
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Mute sound from other clips while recording: Click the Voiceover Options button and select the Mute Project checkbox.
Record a voiceover - iMovie Help
All my best