Your question is vague. There are no simple functions nor SDK to convert audio from sound card into AM/FM/USB I/Q modulated signal. This is SDR technology with all benefits over analog systems but at a higher cost in requested intellectual knowledge (about software and hardware).
If you want to learn how SDR works find open source projects using sound card as A/D and D/A converters (PowerSDR for example). Each program has a library for DSP functions (DttSP.dll) where all I/Q modulation-demodulation are performed. Once you fully understand I/Q data system just change the sample rate to LimeSDR range (48k-192k audio into Mb/s) and voila: you are transmitting! For a simple modulation scheme forget FPGA usage (only needed for GPIO read/write access).
PowerSDR has software port for Red pitaya, the principle operation for LimeSDR is identical except a difference in sample rate.
Goran