You can get an idea from the power and voltage at impedance figures since at 50 mV THD+N is strongly dominated by noise. From the latter it seems to be around 81 dB.

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Since 50 mV across a 300 Ohm load gives a power of 0.05^2/300 = 8.33*10^(-6) W = 8.33 uW, the 300 Ohm power figure gives a slightly lower value closer to 80 dB.

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Since 50 mV across a 32 Ohm load gives a power of 0.05^2/32 = 7.81*10^(-5) W = 78.1 uW, the 32 Ohm power figure gives a slightly higher value closer to 81 dB.

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Thus, it is probably around 81 dB. According to the specifications, the SNR of the headphone output is 126 dB while the output is 7.5 V. Thus, the SNR at 50 mV should be around 126 - 20*log10(7.5/0.05) = 82.48 dB.