With either rhythmbox-plugins-3.0.2-0ubuntu2 (Linux Mint 17.3 Cinnamon) or rhythmbox-plugins-3.3-1ubuntu7 (Linux Mint 18.2 XFCE) when the "Song Lyrics" plugin is enabled and the following are checked, lyrics are never displayed:
* WinampCN (www.winampcn.com)
* Dark Lyrics (darklyrics.com)
* Jetlyrics (jetlyrics.com)
* I didn't use the other two since I speak only English
Looking at the code in '/usr/lib/rhythmbox/plugins/lyrics/' and experimenting with the queries suggests that the remote web sites have changed.
For example, 'JetlyricsParser.py' has:
* q = title + ' - ' + artist
* url = 'http://www.jetlyrics.com/search.php?q=%s' % (q)
But http://www.jetlyrics.com/search.php?q=acdc-hells+bells is 404, while doing it manually from http://www.jetlyrics.com/ yields http://lyrics.jetmute.com/search.php?q=acdc+hells+bells&search=Search with some results.
Likewise DarkLyricsParser.py has:
* url = 'http://www.darklyrics.com/lyrics/%s/%s.html' % (best_match.artist, best_match.album)
But http://www.darklyrics.com/lyrics/acdc/hells+bells.html returns a 404/search page and the search "acdc hells bells" from that page is null but manually working through the form you can get https://www.azlyrics.com/lyrics/acdc/hellsbells.html.
In those examples both the base web site and search strings have changed, rendering the plugin code out of date.
Also, CTRL-L no longer works in either Mint 17's RB 3.0.2 or Mint 18's 3.0.0, but "View > Song Lyrics" works.
Status changed to 'Confirmed' because the bug affects multiple users.