Whenever the Latin phrase in excelsis occurs in a song (typically in gloria in excelsis), I sing it as "in egg-shell-sis".
That's a pronunciation our music teacher at school taught us at one point, and I've kept it up ever since. (Regardless of whether the remainder of the song is in Latin, German, or English.)
He also wanted us to sing "gloria" as (roughly) "glaw-dee-a", but I think that was mostly to avoid having an overly rhotic vowel as in "glorrrrrr-ee-a". Since my default speech isn't rhotic, I didn't pay that much attention to that bit.