FFmpeg concat and then add soundtrack results in a soundtrack that stutters - ffmpeg

Question

I am trying to create a video composed of clips of images and videos. For the clips of images, I use ffmpeg to create a video file and then I add a silent audio stream through these two steps:
ffmpeg.exe -loop 1 -i MyImage.png -codec:v libx264 -t 4.0 -profile:v high -preset slow -r 25 -b:v 500k -maxrate 500k -pix_fmt yuv420p -vf scale=1280:720 MyImageMovie.mp4
ffmpeg.exe -f lavfi -i anullsrc=r=48000 -i MyImageMovie.mp4 -shortest -c:v copy -c:a aac -strict experimental -y MyImageMovieWithSilentAudioStream.mp4
Then I combine my video clips and image clips with
ffmpeg.exe -f concat -i videoList.txt -c copy -y concatVideo.mp4
At this point, the video looks good, any video clips that have audio streams seemed well synced to the video.
Now I add a soundtrack:
ffmpeg.exe -i concatVideo.mp4 -i soundtrack.mp3 -ar 48000 -filter_complex "[1:a]apad [b] ; [0:a][b]amerge=inputs=2[a]" -map 0:v -map "[a]" -c:v copy -ac 2 -shortest -y FinalVideo.mp4
The problem is that the soundtrack on FinalVideo.mp4 stutters at some (not all) of the concatenation joints.
I suspect it has to do with the audio stream and the video stream of the Image clips not being perfectly aligned. The aac has .0231s resolution and the video has 0.04s resolution. When I ffprobe the MyImageMovieWithSilentAudioStream.mp4 the duration is 4.00s but the start is 0.0213.
If my concatenated video has several of these image clips, the error can start to accumulate.
What can I do to keep the video and audio in sync and add a soundtrack that doesn't stutter?
Also, this is a little interesting, I don't hear the stutter when the final video is played on Windows Media Player, but it is there if I play it on VLC or via the html native video element.

Answer 1

Try adding the soundtrack in the same step as the concat.
ffmpeg -f concat -i videoList.txt -i soundtrack.mp3 \
-filter_complex "[1:a]apad[b];[0:a][b]amerge=inputs=2[a]" \
-map 0:v -map "[a]"
-c:v copy -c:a aac -ac 2 -ar 48000 -shortest -y FinalVideo.mp4
As an aside, you can also combine the image and silent stream generation,
ffmpeg -loop 1 -i MyImage.png -f lavfi -i anullsrc=r=48000 \
-vf scale=1280:720 \
-c:v libx264 -profile:v high -preset slow -r 25 -b:v 500k -maxrate 500k -pix_fmt yuv420p \
-c:a aac -strict experimental -t 4 -y MyImageMovieWithSilentAudioStream.mp4