Well, it seems that I've stalled the thread, so here's the answer: The lips.
The Fëanorian Letters - Appendix E (i), and the illustration of the Tengwar - RotK
'the first line is for the teeth' - Series 1 is for the dental series: t, d, etc
'the second for the lips' - Series 2 for the labials: p,b, etc
'the third and fourth vary' - Series 3/4 "usually represent aspirated consonants...but might represent other..variations required"
'the fifth is for the nose' - Grade 5 is for the nasal consonants: n, m, etc
'the sixth line is for the weakest' - "..most often used for the weakest or 'semi-vocalic' consonants": untrilled "r", etc