Whether cash vouching is part of internal auditing

Cash and bank verification is a part of audit.

The scope of internal auditing is defined by management. If they cover then it has to be done

Internal Audit Covers Everything, Ofcourse Scope defined by Management

In real it covers all accounting and Non accounting areas too

Yes. cash vouching is also a part of internal audit. The internal auditor should verify that the cash paymnets have been authorized. He should also verify that the vouchers are supported by proper bills. Further, the auditor should also check whether revenue stamps have been affixed on paymnets exceeding Rs. 5,000/-.

Also, comliance with Section 40A(3) & Section 40A(3A) should be checked to ensure that payments exceeding Rs. 20,000/- have not been made.

I agree with ashish, 

If it forms part of your audit scope then you to verify otherwise you are nowhere obliged to do so, if the same does not forms part of your audit scope.