(Amplitude modulation - Wikipedia)
If you want 100% modulation, then the “message” needs to have a range of 0 to 1.
If this is about “Silent Subliminals”, then this topic may help: Silent Subliminals [solved?]
Also, note that sound cards are designed for humanly audible sound, so they are often limited to a maximum analog in / out frequency of not much more than 20 kHz (even if they support sample rates of 192 kHz or more).