This is what really matters. It confirms that the original WMA file had seek accuracy problems since an export from WMA originally didn’t work. “Convert to Edit-Friendly” will be a necessary step if continuing to use WMA files.
It means your computer is struggling to play these large video files. When playing a section of video, Shotcut can’t decode video fast enough (or the hard drive can’t supply data fast enough), and the audio glitches as a result. However, if the same section is played again, the video is likely coming out of the frame cache in RAM and will appear to work fine until playback reaches a part of the video that isn’t in cache. Then the glitching starts again.
If the data rate from the hard drive is part of the problem, then this can be reduced by using the “Medium” option of Convert to Edit-Friendly. You will be unable to tell a visual difference in quality compared to High.
If that doesn’t provide a full fix, then Proxy files and Preview Scaling are the usual solutions.