Regulus gave a self-deprecating chuckle at Sirius' mention of them not being on speaking terms, and he leaned back and stroked his chin as he listened intently to the words. The song captured their old dynamic quite well, and Regulus thought he could easily voice the "Nothing is easy with you" line, too.
He knew it probably wasn't easy for Sirius to put things out there so openly; certainly, it wouldn't have been easy for him, and he didn't think they were that different when it came to exposing their vulnerable sides. Thus rather than taking offense, he admired Sirius for taking such a risk, and between the end of that song and the beginning of the next, he applauded enthusiastically.