How to convert volts to dB SPL
DB SPL is a pressure measuring unit.  

You can't convert a voltage to an DB SPL reading unless you know:


the microphone sensitivity (or simply the analog output voltage to the input pressure ratio) which tells you essentially the voltage level it will output for a given sound pressure level
the gain that the preamp has applied




Your microphone has a sensitivity of -46dBV/Pa , this gives 0.005012 V RMS/ Pa
1 Pa (pascal) equals 94 dB sound pressure (SPL)

The dB equation for voltage is \$ 20 \times \log  \frac {V_1}{V_o} \$
where V1 is the voltage being measured, and \$ V_0 \$ the reference level

If we do an example calculation for the measurement of 2.5v (assuming a unity gain for the amplifier) we get

\$ 20 \times \log  \frac {2.5}{0.005012} = 53.96dB \$

so the SPL will be (-46) + 53.96 = 7.95 + 94 = 101.95 Db SPL

We assumed a unity gain for the preamplifier, if the actual gain was 20dB then the SPL becomes
 101.95 - 20 = 81.95 Db SPL

if the actual gain was 10dB then the SPL becomes
 101.95 - 10 = 91.95 Db SPL
...