The Mechilta derabbi Yishmael itself contains this question.
וכי מה אמר המקום למשה לאמר לישראל או מה אמרו ישראל למשה לאמר למקום
What said G-d to say to Israel and what said Israel to Moses to answer to G-d?
We would learn at the end the answer which is written in the Mechilta. But this answer needs an introduction.
Many Rishonim and Acharonim Commentators addresses the question, the following words of the Keli Yakar may help to answer this question and to understand further the answer of the Mechilta. Here is a stuff of the Keli Yakar.
כי היה להם לומר סתם כן נעשה אלא שאמרו כל אשר דבר ה' למעט דברי משה, גם רמזו באמרם כל אשר דבר ה' נעשה.
There was no next words of the people which were hidden by the Tora. The verse 9 does only explain the verse 8.
G-d expected Moses to act as an intermediary, and Moses anticipated the refusal of Israel making a diuk from the previous answer written in the verse 8. In the verse 8, "כֹּ֛ל אֲשֶׁר-דִּבֶּ֥ר יְהוָֹ֖ה נַֽעֲשֶׂ֑ה ". We agree to make every thing that G-d said. From the formulation they did not say simply "we will make this". Here is a diuk: "ולא מה שתדבר אתה" "but not what you would say". Actually the fact that Moses did make this diuk is written in the verse 9 "'וַיַּגֵּ֥ד מֹשֶׁ֛ה אֶת-דִּבְרֵ֥י הָעָ֖ם אֶל-ה". The verse reported the fragment of the diuk by paraphrasing the fragment of sentence "אשר דיבר ה'" in a clearer way "אל ה".
So, there is no new dialog which was hidden, but verse 9 contains a comment of Moses regarding the previous dialog between him and Israel.
And now we can read more pshatic the answer of the Mechilta:
אלא אמרו, רצוננו לשמוע מפי מלכנו, לא דומה שומע מפי פרגוד לשומע מפי המלך
Moses said "They said: Our wish is to listening from our King (G-d). To listening from the emissary is not similar as to listening from the King".
I hope that it makes sense. Without the question it is quite impossible to understand this Mechilta.