Streaming audio from PC to Arduino over WiFi
Well, a sort of... You can stream 64kbps MP3, get it via wifi from esp8266. Then for all the rest, forget the Arduino.

You need a dedicated MP3 decoder chip (many out there, sta013 was one of the oldest beasts) which can take the digital mp3 stream and convert it to analog output.

You can feed the line level analog output to the FM Transmitter, and then you're golden.

Note that you shall develop some application on the PC to actually stream out the MP3 - an off the shelf solution such as a Shoutcast server may not work, as the usual web-radio stations inject station data and currently played track metadata into the mp3 stream what you have to remove from the MP3 decoder chip.

Since the actual wifi bandwidth with ESP is limited (as well as the processing power of the Arduino), you may switch to a more efficient compression format. For example, a 64kbps WMA sounds far better than a 64kbps MP3, and at 64kbps AAC is superior to any of the other formats. Ensure that your decoder chip can work well with a modern codec comperssion format.