Reserve requirements are now abolished in many countries including US. Also a proper answer to this depends on exact region we are talking about since different countries may have different laws but generally speaking:
What can be accounted as reserve (apart from actual reserve balance at a regional central bank) ?
For purposes of the regulation nothing else. Economically speaking any cash or cash equivalent could be used as a reserve, but the reserve requirement regulation did not allow (generally speaking) for reserves to be held outside CB.
how/where do banks file their reserve amount?
At the central bank. Exact bureaucratic process depended on what jurisdiction we are talking about.
who verifies whether this informed amount is correct? Is this amount audited and by who?
This again depended on jurisdiction. In US this would be done by Fed and generally central bank or some bureau for financial regulation would be involved in each country.
what happens if a commercial bank fails to meet its reserve requirement?
They would be fined depending on the size of discrepancy. Potentially the fine could be large enough to force the bank to shut down.