What are “Items” in OS X Photos

1. Navigate to Pictures folder
2. Right click on Photos Librar and Show Package Contents
3. Open the originals folder (prior to macOS 10.15 / Catalina, the 'originals' folder was named 'Masters')
4. In search bar, type mov, then choose Type 'Movie'
5. Make sure 'Masters' is still the search target
6. Right click the column headings and tick 'Size'
7. Click Size column header to sort

Do not change anything in Finder. Just use it to identify the videos, then go back to Photos to make changes.

(Answering my own question.)

The problem is that the Olympus SZ-31MR encodes its audio as signed 16-bit PCM. This audio format is not supported by Photos. So the audio needs to be transcoded in order to play from Photos.

The command-line program ffmpeg can transcode audio streams while leaving the video streams unchanged. For example, here's how to transcode the audio to Apple Lossless (ALAC) and copy the video:

$ ffmpeg -i input.mov -vcodec copy -acodec alac output.mov

Experimentally, it seems that Photos doesn't mind if I edit the videos in its library, so here's a shell script that automates transcoding the Olympus's 16-bit PCM to Photos-compatible ALAC:

#!/bin/sh
# Convert movies in Photos library from 16-bit little-endian PCM to
# Apple Lossless.

set -e

# Location of Photos library
PHOTOS=~/"Pictures/Photos Library.photoslibrary/Masters"

# Directory for movie backups
BACKUPDIR=~/movie-backups
mkdir -p "$BACKUPDIR"

find "$PHOTOS" -name '*.MOV' | while read MOV; do
  if ffprobe -v fatal -show_streams -select_streams a -of flat -i "$MOV" |
    grep -q 'codec_name="pcm_s16le"'
  then
    echo "Transcoding $MOV..."
    BACKUP=$BACKUPDIR/$(echo "$MOV" | sed 's+_+__+g;s+/+_+g')
    TMP=$BACKUPDIR/tmp.mov
    ffmpeg -v fatal -y -nostdin -i "$MOV" -vcodec copy -acodec alac "$TMP"
    mv -- "$MOV" "$BACKUP"
    mv -- "$TMP" "$MOV"
  fi
done

This script is careful to keep the original movies in a backup directory in case something goes wrong, and it uses set -e and ffmpeg -v fatal to avoid continuing after an error, but it does overwrite the movies in the Photos library, so use it at your own risk.