Yes, several founders spoke against it. And some spoke for it. As for Jefferson, these previous posts deal with his [opinion] (https://www.reddit.com/r/AskHistorians/comments/l784qn/did_thomas_jefferson_become_more_proslavery_later/) of [the practice] (https://www.reddit.com/r/AskHistorians/comments/j9574l/thomas_jefferson_wrote_the_declaration_of/). The short there is he did speak against it and even tried to stop it in Virginia, but it's more complicated than that. Some other founders:

John Jay was the son of one of New York's largest slave holders, and he was adamant about ending the practice. He wrote of his desire to end the practice to Dr Benjamin Rush when drafting the NY constitution in the 1770s, and, in 1785, helped form The New York State Society for Promoting the Manumission of Slaves. That group successfully lobbied a NY bill signed in 1799 establishing gradual emancipation and ending the practice gradually - it was signed into law by none other than than Governor John Jay.

Abraham Baldwin, a Connecticut born man who served as a chaplain in the Revolution before relocating to GA and founding the University of Georgia, said at the Constitutional Convention that he believed GA would "put a stop to the evil" if left to her own decisions. He was also instrumental in the famous compromise of that Convention.

Madison knew it was wrong but never pushed for an end to the practice, but did push to end some forms of trading.

Monroe inherited his first enslaved souls at age 16 with the death of his father. He grew his plantation in size and, in the early 1800s, he had several dozen enslaved souls laboring for him. He was number 15 in number of slaves held on the 1810 Albemarle County census with 49; topping him on the county list that same year was Jefferson, who reported holding 147. Monroe was Governor when Gabriel's Rebellion occurred, or was supposed to occur, and it was under his authority that numerous conspirators were hung. It was also under his authority that 8 were pardoned under mercy, instead of being executed being banished from the state. He had a series of letters with TJ covering the mercy debate and later would begin to openly oppose Slavery, instead proposing repatriation to their "homeland," Africa. Monrovia, the capital of Liberia, is named as a result of his actions to resettle enslaved Americans there to prevent the burden and violence inevitable if they were freed and remained in America. He gets half points here.

Hamilton opposed Slavery but not aggressively and would generally not be considered an abolition activist by our standards. He was a member of the NY manumission society, but some members of that group were slave holders. He proposed a vote to kick them out but it failed to pass. Some members, including Jay, would purchase humans and free them after they repaid the cost through service, essentially treating them as indentured servants (and then tenant farmers under an early version of the crop-lein system). Some members were legit slave holders who, like Jefferson, sought large scale emancipation gradually but took no action on their own estates to enact it locally.

Perhaps the most vocal was Governour Morris. During the Constitutional Convention he gave a speech, declaring he would never support domestic Slavery. He said the taking of Africans by Americans was a violation of "the most sacred laws of humanity" and that Slavery delivered those souls into "cruel bondages," calling the whole system a "nefarious institution."

Dr Benjamin Rush opposed the practice and joined the first American abolition society in 1787, The Pennsylvania Society for Promoting the Abolition of Slavery, which was formed as The Society for the Relief of Free Negroes Unlawfully Held in Bondage but renamed after the death of its founder, Anthony Benezet. Rush once said, "...all of the claims of superiority of the whites over the blacks, on account of their skin color, are founded alike in ignorance and inhumanity." He argued, publicly, as early as 1773 that blacks and whites were equal - yet he held at least one as late as 1794. He also wrote the constitution for the Philidelphia Society, so we see conflict here, too.

Dr Franklin was rather quiet at the '87 Convention, however he had been asked by the Philadelphia Society to represent their cause. Many believe one of the unnamed items he disagreed with in the Constitution, as he declared in his speech the day it was signed, was the continuance of Slavery. He didn't just represent the Society, he had become its president in '85 following the death of Benezet in '84. His last action in public life was sending the US Congress a petition;

...That mankind are all formed by the same Almighty being, alike objects of his Care & equally designed for the Enjoyment of Happiness the Christian Religion teaches us to believe & the Political Creed of America fully coincides with the Position. Your Memorialists, particularly engaged in attending to the Distresses arising from Slavery, believe it their indispensable Duty to present this Subject to your notice. They have observed with great Satisfaction that many important & salutary Powers are vested in you for "promoting the Welfare & Securing the blessings of liberty to the "People of the United States." And as they conceive, that these blessings ought rightfully to be administered, without distinction of Colour, to all descriptions of People, so they indulge themselves in the pleasing expectation, that nothing, which can be done for the relive of the unhappy objects of their care, will be either omitted or delayed.

From a persuasion that equal liberty was originally the Portion, It is still the Birthright of all men, & influenced by the strong ties of Humanity & the Principles of their Institution, your Memorialists conceive themselves bound to use all justifiable endeavours to loosen the bounds of Slavery and promote a general Enjoyment of the blessings of Freedom. Under these Impressions they earnestly entreat your serious attention to the Subject of Slavery, that you will be pleased to countenance the Restoration of liberty to those unhappy Men, who alone, in this land of Freedom, are degraded into perpetual Bondage, and who, amidst the general Joy of surrounding Freemen, are groaning in Servile Subjection, that you will devise means for removing this Inconsistency from the Character of the American People, that you will promote mercy and Justice towards this distressed Race, & that you will Step to the very verge of the Powers vested in you for discouraging every Species of Traffick in the Persons of our fellow men. B. Franklin, Feb 3, 1790

More can always be said but this older answer might be of interest.