When joining a letter with a Kasrah or Dhammah to a letter with a sukoon, what is the correct sound for the vowel?

Question
Assalamu'alaykum wa rahmatullahi wa barakaatuh

in shaa'Allah you are well and in the highest of Eemaan. When joining a letter with a Kasrah or Dhammah to a letter with a sukoon, what is the correct sound for the vowel? From what I understand; because it is joined, the sound is not like a  Kasrah or Dhammah when read on its own (without joining) as one will have to keep the mouth in a neutral position for the saakin letter. Can you please clarify this for me.

JazakAllahu Khairan

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
Al-hamdu lillah we are well and we ask Allah to increase us in eemaan daily and you as well. 
The sound of a kasrah is the same sound of the lengthened ya', but shorter.  The sound should not change from when there is a kasrah before a voweled letter and a kasrah before a saakin letter.  Both should sound like a shortened medd letter ya'.  This takes practice to achieve when the letter after the kasrah is saakin.  We have to take care to retract our jaw and also use the middle of the tongue. 
The same holds true for the dhammah.  All dhammahs should sound alike whether before a voweled letter or a sakkin letter.  The dhammah is a shorter version of the lengthened wow and should have the same sound, but briefer.  We need to take care that our lips are circled and straight out from our mouth during the dhammah. 
After we pronounce the kasrah or dhammah with a full harakah (one full count) then we put our mouth in a neutral position for the subsequent saakin letter.  Some students mistakenly hurry the kasrah or dhammah trying too quickly to go to the neutral position. 
All harakaat (vowels) should be equal in timing and all dhammahs should sound alike and all kasrahs should sound alike. 
Wa iyyakum. 
Baarak Allahu feekum