What about if we hear...

1. yi,ye,ya,ying,yang

2. wu,wo,wan,wen

3. yu,yue,yuan,yun

i will be accompanied by "y" or if the initial is needed 'i' will be replaced by 'y'
u will be accompanied by "w" or if the initial is needed 'u' will be replaced by 'w'
ü will be written as 'u' when the preceeds are "j,q,x,y"

i----yi
ie----ye
ia----ya
ing-----ying
iang----yang

u----wu
uo----wo
uan----wan
uen----wen

ü----yu
üe----yue
üan----yuan
ün----yun