Would you explain the output waveform of this FM transmitter circuit?
Why someone hasn't answered this beats me....

Your sound card input is good for 20 kHz and maybe 80 kHz on a good sound card sampling at 196 kHz. 88 MHz is well over a thousand times too high in frequency.

Look at the time base dial on the scope picture - it goes down to 1 ms and, in 1 milli second 88,000 cycles of FM carrier would have occured.