Suppose the last song is the duet during the ending credits, is the song you seek the concert beginning with empty seats in an abandoned concert hall?
Tenshi no Enogu
Sung by Mari Iijima, but I also have an mp3 with the title and some name called Sakurai Tomo, it's a slightly different version, but also sung by Mari if my ears are not decieving me.
The Sakurai Tomo (she's the speaking voice of Mylene, one of the Milky Dolls, and Canary Minmay) version of Tenshi no Enogu, if I'm not mistaken, is from the Macross Generations radio drama. In the Macross Universe, it's sung by Canary Minmay, a singer on the Macross 9 fleet who makes a living covering Minmay songs.
Learning new things every day!