The tape recorded the conversation between Yan Chen and Qiu Tu on the night of Jia Shu's death.
In the conversation, Qiu Tu explicitly mentioned that Jia Shu could be in danger, and Liu Xiongyuan might be preparing to ambush him.
He also suggested that Yan Chen should go to rescue Jia Shu.
However... after hesitating, Yan Chen chose to forgo the rescue of Jia Shu and even... didn't alert Jia Shu.
This decision ultimately led to Jia Shu being killed by Liu Xiongyuan.
Since the recording was made secretly, and it couldn't be confirmed that the voices were indeed those of Qiu Tu and Yan Chen—they could be two people with similar voices—it couldn't be taken as genuine evidence.
Even if, given the statuses of Huang Shangzong, Liu Xiongyuan, Yan Chen, and Qiu Tu, the recording could be accepted as evidence, it would still be ineffective.