The only sound file type Tumblr supports for uploading is MP3. This means that using a YouTube video as the external URL for an audio file on Tumblr will not work. The post will publish, but you'll receive an error upon attempting playback. However, you can convert a YouTube video to a sound file quickly and easily.

Convert YouTube to MP3

To convert a YouTube video to an MP3 file so that the audio from the video will play only as sound on Tumblr, first find the YouTube video you want to share. Highlight the URL of the video, right-click, and choose "Copy." Then visit YouTube MP3, a free online website that converts YouTube videos to MP3 files, right-click the text field and choose "Paste" to paste in the video's URL. Click "Convert Video" and then "Download." Once you've downloaded the audio from the YouTube video, upload that MP3 file to Tumblr by clicking the "Audio" icon on your dashboard and choosing the "Upload" tab.