"Despite having proper telephonic intercepts as corroborative evidence to prove Dawood's involvement in running the whole betting syndicate, we need some more audio clips of his voice for the authenticity," an official, who did not wish to be named, told IANS.
Investigators were awaiting a forensic report on the taped conversations as it would bolster their case against the accused, he said.
The officer said the tapes had voices of bookies, players and other people, who were handling spot fixing and betting from abroad, and police need the Delhi Forensic Science Laboratory (DFSL) report to clarify the roles of the arrested accused.
"We have sent the tapes to DFSL along with the voice and hand writing samples of the arrested bookies and players. The forensic officials will take some time to give the report," said the official.
Police have telephonic intercepts in which some of the bookies made calls to Dubai, Karachi and other cities of Pakistan to fix and alter spot fixing and betting rates, said the official.
"The calls were made to fugitive don Dawood Ibrahim and his aide Chhota Shakeel, whose name is mentioned in the list of suspects in the IPL spot fixing case," said the official.
The underworld don's name came to light after the arrests of bookie Ramesh Vyas, in Mumbai by Mumbai police, and bookie Ashwani Aggarwal alias Tinku Mandy, in Mumbai by Delhi Police, last month.
Vyas and Aggarwal were allegedly handling Dawood Ibrahim's betting syndicate in south and north India.
Delhi Police moved an application before a city court June 7 seeking open non-bailable warrants (NBWs) against Dawood Ibrahim and Shakeel.
"It has come to our notice that the main bookies and fixers were in touch with Dawood Ibrahim and Chhota Shakeel...who gave directions to bookies and fixers in India...," the official said.
Delhi Police have so far arrested 26 people, 21 of these - including Rajasthan Royals players S. Sreesanth, Ankeet Chavan and Ajit Chandila - have got bail. Bookie Aggarwal is in Mumbai police custody.
All the 26 people arrested by Delhi Police have been booked under the stringent Maharashtra Control of Organised Crime Act (MCOCA).
Altogether, 60 people have been arrested across the country in connection with the IPL spot fixing scandal.