Yes audio recording is admissible in evidence but it needs to be in a CD and with a certificate issues by person who writes it into CD. Please refer section mentioned below.
Section 65B in The Indian Evidence Act, 1872
1[65B. Admissibility of electronic records.—
(1) Notwithstanding anything contained in this Act, any information contained in an electronic record which is printed on a paper, stored, recorded or copied in optical or magnetic media produced by a computer (hereinafter referred to as the computer output) shall be deemed to be also a document, if the conditions mentioned in this section are satisfied in relation to the information and computer in question and shall be admissible in any proceedings, without further proof or production of the original, as evidence of any contents of the original or of any fact stated therein of which direct evidence would be admissible.
(2) The conditions referred to in sub-section (1) in respect of a computer output shall be the following, namely:—
(a) the computer output containing the information was produced by the computer during the period over which the computer was used regularly to store or process information for the purposes of any activities regularly carried on over that period by the person having lawful control over the use of the computer;
(b) during the said period, information of the kind contained in the electronic record or of the kind from which the information so contained is derived was regularly fed into the computer in the ordinary course of the said activities;
(c) throughout the material part of the said period, the computer was operating properly or, if not, then in respect of any period in which it was not operating properly or was out of operation during that part of the period, was not such as to affect the electronic record or the accuracy of its contents; and
(d) the information contained in the electronic record reproduces or is derived from such information fed into the computer in the ordinary course of the said activities.
If accused denies it's not his voice you can seek court to take help of FSL to prove its his voice if you have filed civil suit get court commissioner appointed.
If you have anything in writing to show he took money from you and signed by him you can file for recovery or if he has advanced any blank cheque get it dishonored and file a suit