Я не использовал vala и не проверял это, похоже, вам нужно заменить
errorBox.setWarning("<span weight=\"bold\" size=\"larger\">" + _("Loading similar songs") + "</span>\n\n" + _("BeatBox is loading songs similar to") + " <b>" + lm.media_info.media.title.replace("&", "&") + "</b> by <b>" + lm.media_info.media.artist.replace("&", "&") + "</b> " + _("..."), null);
на
string title = lm.media_info.media.title.replace("&", "&");
string artist = lm.media_info.media.artist.replace("&", "&");
errorBox.setWarning(@"<span weight=\"bold\" size=\"larger\">Loading similar songs</span>\n\n BeatBox is loading songs similar to<b> $title </b>by<b> $artist </b>...", null);