You cannot do this without either (A) a lot of information about the amplifier circuit and the speaker, or else (B) a calibrated microphone whose signal you are reading in and using to adjust the values of your output.
The output to your sound card is just going to be a bunch of digital bits. The output of your sound card is just going to be an analog voltage signal with a fairly nominal power level such as 0.3V peak to peak.
Neither the bits nor the output voltage are going to be sound waves: you need at least a speaker for that, and fairly likely an amplifier to reach 10 dB.
Recall, though, that amplifiers and speakers almost never have "flat" response to all frequencies, so you are probably going to have to pick a frequency and work with it -- unless, that is, your 10 dB should refer to the total Sound Pressure Level, and even then there isn't much point emitting frequencies that are going to break the speaker or get cut off by the safeties built in to the speakers.