Revelation 5:
 8.  And when he had taken the book, the four beasts and four and twenty elders fell down before the Lamb, having every one of them harps, and golden vials full of odours, which are the prayers of saints.
 9.  And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;
 10.  And hast made us unto our God kings and priests: and we shall reign on the earth.
 
Gary B.
 
You are correct that it says that the four beasts and the four and twenty elders are singing this new song and does not say that the four and twenty elders are the church but it does give information from which we can make determinations of at least who they are not. They have been redeemed by the blood of the Lamb so they are humans not angels, they are from every kindred, tongue, people, and nation, so they are not Jews or Israelites only so they are not Jesus' disciples, they have been made kings and priests and will (still future at this point) reign on earth, they are not the saints that are still on earth for they have golden vials full of odours, which are the prayers of saints so there are still saints on earth at this point who are praying but once you are in heaven you no longer need to pray for yourself... If this is not the church, then who?
 
As for when the 144,000 will be preaching, it is relatively recent that a view known as "pre wrath" rapture has been gaining acceptance. The view held more commonly before this view and the difference between the two was that the rapture was believed to have to take place before any of Daniel's 70th week (a 7 year period unless it is shortened) would begin at all and this would allow for the 144,000 to be preaching along with the two witnesses any time during the final 7 years because it used to all be viewed as God's wrath and Jacob's trouble which we as Jesus' church were not appointed to but raptured before any of it could begin. However with the view called "pre wrath" the church remains here for at least some part of Daniel's 70th week but just not the "wrath" part. In this view we are seen as possibly already having entered the final 7 years now and await the rapture to happen any time now prior to the actual wrath of God being poured out. So you are pointing out that the 144,000 are sealed just prior to the wrath being poured out and therefore are here for the last half. I just find it possible that the entire time of Jacob's trouble may be wrath which we are not appointed to any of and that none of it has begun yet. The difficulty with the "pre wrath" view, which suggests that we are currently nearly through the first 3.5 years of the 70th week, in my opinion, is that it greatly reduces the time period for the events described in Revelation to all fit into a shrinking time period because we have as yet seen NONE of the things going on that are supposed to be happening during the final 7 years known as Jacob's trouble. No 144,000, no two witnesses, no "beast out of the bottomless pit-Antichrist," no false prophet, or any of the other horrible things described and it begins to seem hard to imagine all of these things taking place in just the last 3.5 years when you place them all together as happening together as Patti C. pointed out, how can it be? How can the two witnesses preach for 3.5 years then be killed by AC and the whole world rejoice over their dead bodies and send gifts to one another at the END when the whole world is completely destroyed? The description sounds too much like a relatively stable world system is in place when the end of their preaching has come for the whole world to look on their dead bodies (on TV, Internet) and then they are called up to heaven and the world fears. None of this has happened yet but it gets difficult for me to picture this fitting into the last half of the tribulation period when you consider all the other things that will be going on then. More likely to me none of it has begun yet and this allows for the entire period of Jacob's trouble to be yet future and gives ample time for the two witnesses to preach for 3.5 years and still be a stable enough world to look on them and send gifts and fulfill the whole prophecy concerning them. We will be in heaven regardless when all this takes place. Hooray! Meeee Meeee Meeee Meeee.... (That's me warming up my singing voice getting ready to sing that new song, I can hardly wait!)
 
Therefore, it makes me tend to go back to the previous belief that we have as yet not entered any part of the 70th week of Daniel and our assumptions about the beginnings of the final 7 years having already begun must be wrong. In summation, to me all of the 70th week of Daniel could be wrath not just the last half and therefore the 144,000 could be preaching during the first half and not limited to only the last half.