Conversion between the two formats seems to be easy. MP4 file format contains both video and audio, whereas MP3 file format only contains audio. With FVC Free Online Video Converter, you can easily convert MP4 to MP3 or change MP3 audio to MP4 without downloading a package.

Step 1. Click Add Files to Convert button from the official page to download and install the launcher.

Step 2. Select the MP4 or MP3 file from the local computer. You can add more than one MP4 files at the same time.

Step 3. Choose MP3 as the output audio or video format at the bottom. Then click Convert button to convert MP4 to MP3 immediately. (Follow the same step to convert MP3 to MP4)

FAQs of MP3 VS MP4： https://www.free-videoconverter.net/tips/mp3-vs-mp4/#part3

Thanks for sharing the information. Honestly, I always follow all the new musical compositions and videos on social networks. Indeed, it is hurt to lose certain data that you laid in bookmarks on the Internet. I prefer to convert all the music I like into the offline mp3 format. I use this program https://mp3convert.cc that helps me to load everything from Instagram, YouTube, or Facebook. It is really easy and fast. So my list of favorite music doesn't suffer and have the access to it anytime. I would advise you also to try it if the other app doesn't work so well.