The discovery was made in Shropshire before Christmas when the piano's new owners had it retuned and repaired.
The collection consists of an undisclosed number of full and half sovereigns, dating from 1847 to 1915.
Experts believe the coins may have been "deliberately hidden" and could yield a "life-changing" sum of money, with a full sovereign worth at least Â£220.
More on this story and others from across Shropshire
An inquest, which will decide whether the discovery can be declared treasure, resumed on Thursday.
Shrewsbury Coroner's Court heard senior coroner John Ellery recount how the piano, made by Broadwood & Sons of London, was initially sold to Messrs Beavan and Mothersole of Saffron Walden, Essex, in 1906.
But its ownership from then until 1983 - when it was purchased by a family in the area who later moved to Shropshire - is unknown.
The coroner is now seeking information about its whereabouts between 1906 and 1983.
Mr Ellery deferred the conclusion of the inquest to allow for more time for anyone with information about the piano from the Essex area to come forward.
The hoard was initially reported to Peter Reavill, finds liaison officer for the British Museum's Portable Antiquities Scheme at Shropshire Museums.
While not declaring the true value of the coins, he said it had "the potential of yielding a life-changing sum of money".
"It's not the sort of money you would tuck away and forget," he said. "It is a lifetime of savings and it's beyond most people."
Alexandra Whittaker, of auctioneers Fellows & Son, said a full sovereign with a 22-carat weight would be worth about Â£220, with a half worth Â£113.
"But if one was particularly special, like if it had something wrong with it, or there were fewer minted that year, then it would be worth a lot more," she added.
The objects will qualify as treasure and be the property of the Crown if the coroner finds they have been hidden with the intent of future recovery.
However, if the original owner or their heirs can establish their title to the find, the Crown's claim will be void.
The inquest will resume and conclude on 20 April.
A hoard of gold discovered hidden inside an old piano has been revealed to be a collection of sovereign coins.