Is it illegal to transmit in the FM range for personal use
Different locations will have different rules. If you live in India, check the regulations for India.

Generally, powerful transmissions in licensed bands like FM audio must be licensed, to prevent interference with other users. If other users complain, you may get tracked down, and your equipment confiscated or destroyed.

Most places have unlicensed bands as well, like the so called ISM frequencies, and usually a few other bands for other purposes as well, like garage door openers and short range personal comms.

If your transmitter is very low power, and operated at times and at frequencies when it could not interfere with any stations your neighbours in range are tuned to, then you can probably figure out what the actual chances of causing trouble or getting caught are.