The simple FM transmitter is too simple:
1) Its tuned circuit is connected directly to the antenna. So if anything gets near it then its frequency will change.
2) It doesn't have a voltage regulator. Then its frequency will also change as its battery voltage runs down.
3) Its input sensitivity is about 5mV. The output from your sound card is much too high and an attenuator is needed.
4) It doesn't have pre-emphasis (treble boost) like FM radio stations have so it will sound very muffled (without treble audio frequencies) when heard on an FM radio.
5) It has a very weak output power and might not have a range to across the street.