I suppose that works to just cobble something together. Still,
though, it will do nobody any good to be learning Lojban from a voice
of this quality; we'll all sound horrible the first time we actually
try to speak to another human. ;-) Which underscores the fact that
it would be really nice to have a high-quality, Lojban-specific set of
diphone recordings, which some kind folks in Brussels have recently
volunteered to do. If you can get your diphone list to them and give
them a hand in figuring out what to do, since you tried it once
already, it might make things easier for them. And you have confirmed
that you've already done all the work that I volunteered to do!
A few notes here. First, English vowels (esp. American English) are
very different from Lojban vowels, and for that matter from most other
languages. English vowels are not pure vowel sounds, but slide from
one vowel to another, as in a dipthong. Lojban vowels are pure,
crisp, and short. Hard to communicate the distinction in text, I
suppose.
The Lojban 'r' is allowed to be pronounced in just about any way,
including the standard American untrilled 'r'. I happen to think that
Lojban sounds best with an Irish sort of 'r', but that could just be
because I love listening to Irish accents. ;-) I would advise
against the French "r", as that seems indistinguishable from "xr", at
least to my ear.