Premier Element 12, no video, only sound

Also, This message is about Premiere Pro, but the process also works for Premiere Elements
H.264 will not work inside an AVI wrapper https://community.adobe.com/t5/Premiere-Pro/Can-t-get-PPro-to-accept-video-file/td-p/3450861

>videos are .avi files from my Garmin dash cam

Adding to what Bill said...

AVI is a wrapper, what is inside YOUR wrapper - Exactly what is INSIDE the video you are editing?
Report back with the codec details of your file, use the program below
Free program to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download
- when you analyze your file in MediaInfo and post a screen shot in the forum, do so in TREE view
- post your information IN your message, not as an attachment that someone would have to download

Does your video use a Variable Frame Rate? See https://community.adobe.com/t5/Premiere-Pro/Variable-frame-rate-video-with-Premiere-Pro/td-p/4601935
If yes, use https://handbrake.fr/ open source transcoder/converter to convert to Constant Frame Rate

I'm sure you know that editing programs and viewing programs are not the same.   Without a MediaInfo printout of the file characteristics I will guess that your Garmin records in a variable frame rate.  Premiere Elements was not very good at that because common video cameras record in a fixed frame rate.   You may have to convert your files using something like the (free) Handbrake program.