My question is about ishmaam. In an earlier lesson, you've referred to it as having no sound

Question
Assalaamu alaikum respected teachers,

I hope that Allah grants you the highest parts of Jannatul firdaws for your efforts, Aameen.  My question is about ishmaam.  In an earlier lesson, you've referred to it as having no sound (it can only be seen but not heard), and this is exactly as Imam Shaatibi (may Allah have mercy on him) described it in his poem in the chapter "baabul waqf 3alaa awaakhiril kalim."  But a little further down in his poem (farsh surah nisaa), the imaam says, "wa ishaamu Saadin saakinin qabla daalihee..." until the end of the bayt.  The shaykh is referring to the rule in the qiraa'aat of Hamzah and Kisaa'ee that when the letter Saad saakin precedes the letter daal (ex. in the words Asdaq or tasdeeq), then the letter saad becomes closer to the letter zaay with ishmaam.

But I've listened to a couple of reciters apply this rule (ex. verse 37 of surah Yuunus), and the circling of the lips is clearly heard.  Can you help me understand the difference between the "ishmaam" of this rule, and the ishmaam in verse 11 of Surah Yuusuf?  Because the ishmaam in surah yuusuf cannot be heard, but the ishmaam in surah Yuunus in the qiraa'ah of Hamzah and Kisa'ee is heard.

I apologize for not having the Arabic scripts in any of my quotes, and for any mistakes that may appear in my question.  May Allah reward you immensely.

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
Jazakum Allahu khairan for your kind dua'.  May Allah grant you all khair in this world and the Hereafter and increase you in beneficial knowledge. 
The linguistic definition of  is mixing or blending, as far as the reading of Hafs 'an 'Aasim, the only kind of ishmaam is the mixing of the dhamm of the lips with a sukoon, as pointed out in the beginning of your question.
 In the ten qira'aat, there are other types of mixing of sounds that occur, and one such case is the mixing of the sound of the letter with the letter when there is a saakinah before a daal.  This means that the is not read as a pure , instead it is mixed with the sound of a, so there is tafkheem and itbaaq, but the sound of the is present .  There is however no dhamm or circling of the lips when this is read.   This should be referred to as ishmaam of the , or ishmaam of the into the , not just ishmaam.   Khalaf 'an Hamzah also reads with the ishmaam of the sound of the and when he recites the word   and .

We also have the mixing or "ishmaam" of the sound of the dhammah and kasrah in the word "qeela"  in the some of the ways of recitation of the ten qira'aat. 

There is no need to apologize for not using the Arabic scripts, as mattter of fact, as the site software does not recognize Arabic so we would not be able to read the question if it had the Arabic letters.