What Greg said, plus 'laudEmus' means 'let us praise.' 'LaudAmus' would be 'we praise.'
As for the pronunciation, just be consistent. If you use soft g (j) in plangemus, then use soft g in virginem and soft c before e in scelera (shelera). Likewise soft v (w) for virginem use soft j (y) for jugiter. Either way the g in jugiter would be hard.
[Personally I prefer all soft consonants for singing and all hard consonants for classic period poetry.]
The spelling of Ihesus might be a clue that the j is supposed to be hard (like juggle), otherwise why not spell it Jesus? But spelling wasn't standardized for a long time yet... good luck. Sounds like fun.
-Glenn