It seems that Google is integrating Audio ads and will allow advertisers to purchase the audio ads soon. After logging into Google Accounts, I noticed a new link of Google Audio ADs. However, when I tried to go to that link, it returned a page which said UNDER CONSTRUCTION.
So, we can expect that Google will be releasing the audio ads in some time and will give option of earning money to the publishers :D
[...] if they are planning to monetize through Google Adsense or may be Google Audio ads in future then they must read the policies properly otherwise, it will be difficult for them to [...]