Clips Altering Pitch Of Audio On Timeline

I am having an issue with the pitch of my clips being played back almost a semitone lower than their intended pitch.

I am editing a series of videos in Final Cut Pro X (v10.4). Each video consists of three of my previously individually exported mp4 clips: A main guitar lesson, bookended by an intro sting and a closing advertisement for my website.

Each individual clip contains music and was exported in exactly the same manner (MPEG-4; AAC, H.264; 1920 x 1080).

However, whenever I drop the 6.2 second intro sting onto the timeline, the pitch of the other clips is dropped by almost a semitone, rendering the guitar lesson content useless, as it’s in the wrong key.

If I remove the offending 6.2 second clip, the pitch is still affected, so I have to delete all of my cuts and start over. I have tried to troubleshoot this issue myself, but have undone hours of work in the process.

Each clip plays at the correct pitch individually, outside of the timeline.

The issue appears to be related to my 6.2 second intro sting. The clip consists of a jpg photo with a Ken Burns transition, accompanied by a short mp3 of music and some overlaying text. I considered that the issue may have been the absence of an actual video, so I dropped a 7 second video onto the timeline, but the clip still affects the pitch of my audio. I tried removing the Ken Burns transition from the clip; removing the audio from the clip; the text from the clip…even the photo from the clip, but there is no change to the pitch of the the subsequent clips.

I have tried multiple restarts; replicating the sting from scratch in a test library; increasing the length of the sting, but the problem remains and is utterly maddening.

My film editing skills do not extend beyond that of making music tutorials. However, I have edited hundreds of videos using Final Cut Pro X and this is the first time I have experienced this issue.

Any help would be very greatly appreciated.

Thank you in advance.