Vowel adjacencies: /iy/ + /ih/, /uh/ + /uw/ + /ux/, /ae/ + /aa/
Closure reduction: /b,d,g,p,t,k/ + /{= b,d,g,p,t,k}cl/
Nasal and liquid expansion: /en,em,el,er/ --> schwa + /{= n,m,l,r}/
It is also possible to form reductions which depend on the
differences between U.S. and U.K. English; for example,
/hh/ is a glide in U.S. English, but also a weak fricative
in U.K. English. Either way, /hh/ might be conflated with
the /hv/ phoneme. The flap /dx/ can be conflated with /d/,
and the nasal flap /nx/ can similarly be conflated with /n/.
I think the number of biphones scored by an instructional
system might ought to be proportional to the learner's
estimated vocabulary in the language being conversed.
Sincere regards,
:James Salsman