Is recording a voice an offence?

Yes... Definetely it is an offence....But it should be seen that the pupose of such act.....means if it is for good faith or not...... tell me details.. then only we can conclude

One of our colleage has heart problem. One fine day, she was not able to breath or speak & my friend called the nurse for the help saying that the patient is serious. After attending the patient, nurse called the person who had intimated her & told her that she was not serious & she should not had called her using the word seriousness.

My friend told her that the she cann't judge the severity of the patient as she is not doctor and the call was recored for evidence.

then,, the nurse is going to do what with that recording......NOTHING

Yes, recording someone's voice or video is an offence without his / her permission. It dpends upon the situation when someone's voice recorded and in which matter.