There is no way to renew MixRadio at the moment.
My free 3 months subscription expired a fortnight ago. I have been to 3 Nokia Care stores in Bangalore to buy a voucher to renew my subscription, but none of them stock vouchers now. Also, the Oxicash link for renewing the subscription online isn't working. The staff at the Nokia Care stores are clueless as to when the vouchers will be made available and give some vague response. From what I read in the Nokia forums, this problem started after Microsoft's acquisition of Nokia.
I feel that the reason the vouchers are no longer being sold is because MixRadio is being spun off into a separate company on the lines of Saavn. Even so, until that is done, Microsoft should have continued to issue the vouchers for MixRadio.