With the first question of the special election edition of Question Time, Mr Miliband was challenged over the note left by former Labour chief secretary to the Treasury Liam Byrne for his successor, which said "there's no money".
He said the note was David Cameron's "regular prop" and that it was his party's "mission" to eliminate the deficit.
Citing comments by shadow chancellor Ed Balls that Mr Byrne's note had been a joke, an audience member said "running a business is anything but a joke", saying someone working in the private sector would be fired if they did the same.
Mr Miliband said his party was "deadly serious" about balancing the books.
He was also challenged by a man in the audience who said it was "ludicrous" he did not accept Labour had spent too much in government.
Labour had "got it wrong" on bank regulation, Mr Miliband said, adding that his party did not "do enough on apprenticeships" or on a "modern industrial strategy".
Speaking on Friday, Labour's general election co-ordinator Douglas Alexander accepted Mr Byrne's note had "not been wise" but said it was the "global financial crisis" which had caused the deficit to multiply, not the actions of the last government.
"Lehman Brothers did not collapse because Gordon Brown built too many schools and hospitals," he told BBC Radio 4's Today Programme.
Mr Alexander, who risks losing his own Paisley and Renfrewshire South seat to the SNP, said it would be a "disaster for Scotland" if the nationalists swept the board, suggesting it would let the Conservatives back into Downing Street.
"The SNP does not want to help the Labour Party, it wants to harm the Labour Party," he said, adding that his party would "not trade away the safety of the country" in any post-election discussions.
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