Is there any way to enable netflow on a serial interface which a part of a Multilink interface. Serial interfaces has no IP addresses.
Even if you could enable netflow on individual interfaces, the traffic is all accounted in a single cache.
So you would be no better off: you'd still not be able to correlate individual bandwidth usage.
You need Flexible NetFlow for that, because FNF enables you to deploy independent flow monitors on each interface, each with their own cache.