I sent Rav Frank’s answer to Rabbi Laufer @ Ask the Rabbi & received this
March 8, 2011 11:39AM
Submitted by: Rabbi Reuven Lauffer
I saw the excellent and well researched answer of Rabbi Frank. Interestingly enough, until today, the Yemenites still refer to it as Sefer Tillim (without the ‘heh’). The only thing that is missing is a definitive reason (not theories) as to why it is called Sefer Tehillim and not Tehillos which would seem to be grammatically correct.