Ecclesiastes 9:6
Also their love, and their hatred, and their envy, is now perished;
neither have they any more a portion for ever in any thing that is done under the sun.
Above is used to say the dead has nothing to do with anything done in this life by others, thus they can not hear us.
I understand that the context of the above verse is visible when you look at verse 4-5 and 7.
The idea of stressing that your only hope / chance of responding to God grace and carrying out good works is before you die. It's too late when you have died.
4
For to him that is joined to all the living there is hope: for a living dog is better than a dead lion.
5 For the living know that they shall die:but the dead know not any thing,
neither have they any more a reward; for the memory of them is forgotten.
The idea that "the dead know not anything" needs to be interpreted with a consistency or consideration of the whole of scripture which has lots of examples where those in heaven do know something and speak etc...(i.e. Rev6:9-10)
Accordingly I understand the idea is that verse 5 is stressing the inability to effect the good works Referred to in verse 7
"...for God
now accepteth thy works."
Perhaps consider also wisdom3:1-3
(Deuterocanon)
Hebrews 12, cloud of witnesses is used to say those in heaven can hear us. However, this text refers to the testimonies, witness of those in Hebrews 11 when read in context.
Read this in light of later verses 22-23"..but you have come to mount Zion and to the city of the living God, heavenly Jerusalem.....
and to the spirits of just men made perfect"(see also col1:12, "..saints in light")
Consider also these saints in heaven as described in bold below
Luke 20:35-36
But they which shall be accounted worthy to obtain that world, and the resurrection from the dead, neither marry, nor are given in marriage:
Neither can they die any more:
for they are equal unto the angels; and are the children of God, being the children of the resurrection.
Rev 6:9-10 shows awareness of events on earth.
Also consider
Revelation 5:13
And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them,
heard I saying, Blessing, and honour, and glory, and power, be unto him that sitteth upon the throne, and unto the Lamb for ever and ever.