This post indicates that the earliest source for the story of Serach bas Asher and the harp, wherein she notified Yakov that Yosef was still alive in a very subtle musical fashion, is a very late midrash known as the sefer hayashar which the author dates to the 16th century. I would like to know: Are there are any earlier sources for the story?
If there is an earlier source, I am not aware of it; it is not among any of the following:
Targum Yonasan ben Uziel Braishis 46:17 says that Serach was the one who told Yaakov that Yosef was alive. There is no mention there of a harp.
Rashi Shmuel2 20:19 mentions in the name of Medrash Agada that Serach was the one who told Yaakov that Yosef was alive. Again there is no mention there of a harp.
Sefer Mayim Rabim in the name of Pirkei Drav Eliezer says that she told Yaakov in a song, - שהיתה מנגנת ובנגון היתה מארכת ומבלעת - again there is no mention of a harp.