D
Через let все должно работать.
а вот
(set-process-sentinel
(start-process "youtube-duration" duration-buffer
"ytdl" "--no-color" "--get-duration" link)
`(lambda (process _change)
(when (eq 0 (process-exit-status process))
(with-current-buffer ,duration-buffer
(message "\"%s\" duration: %s"
,title (string-trim (buffer-string)))
(kill-buffer)))))