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Unfortunately, SNR cannot be directly compared to NM.
SNR is a relative measure between the desired signal and some arbitrary noise floor that is unknown to us. That noise floor is a function of the receiver's circuitry and can be affected by internal components like filters, amplifiers, automatic gain control, and signal processing stages.
NM is measured relative to the thermal noise floor (Johnson-Nyquist noise), which is the natural background noise of a room temperature receiver, and is essentially a constant value derived from the laws of physics.
Since we don't know what the noise reference is in the SNR calculation, we cannot translate it into a corresponding NM value.
Also, if the Sony SNR estimate is accurate, you'll need at least 15.2 dB of SNR to get a watchable DTV signal. That is the theoretical minimum SNR needed to piece together enough good digital data to reconstruct the original video stream. In practice, you'll want to stay well above that so you have some margin for random fluctuations in the signal.
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