It is a lousy FM transmitter:
1) It is mono, not stereo.
2) It doesn't have pre-emphasis (the treble boost that FM radio stations use}, so it won't have any treble when heard on an FM radio. It will sound like an AM radio or like your stereo with its treble control turned all the way down.
3) Its RF frequency will drift all over the place when something gets near it or near its antenna, when the temperature changes and when the battery voltage changes (as it runs down).