Hello Paul,
The ADAU1761's toolbox includes a Voice Activity Detector (VAD). If you haven't already tried it, perhaps you could press this into service as your beat detector. Its description is at Voice Activity Detector [Analog Devices Wiki] I don't have -1761 hardware to try this out, however I did test a similar idea on the ADAU1701. This chip has no VAD in its toolbox so I made my own some time ago (see VAD for ADAU170x ). I tried a modified version as a beat detector, the signal flow appears below:
With a music signal applied, the circuit blinks a LED on the ADAU1701MINIZ board at peaks in the music. You could instead route this signal to a microcontroller's digital input pin and run an algorithm to figure average beats/min. Music with an obvious disco-like beat (such as Rod Stewart's Forever Young) blinks the LED fairly reliably, less so with a more subtle beat.
The SPL meter is relatively easy to do, simply filter the audio for the A-curve, a roughly approximate filter is shown above. Then use a dynamic envelope follower (with slow or fast dynamics as you choose) to drive a readback block, which your microcontroller can read via I2C and interpret as dB SPL. This should work fine within a reasonable dynamic range (such as 60 dB). For quiet music, you could set your mic gain (etc) for a SPL range of 40 to 100. If instead you're in a "live club music" situation, of course you'll need to calibrate it for a much higher range (for example, 70 to 130 dB).
Be aware that smartphone beat counting and SPL apps are readily available -- depending on your application this may squeeze the economic viability of building a self-contained unit.
Best regards,
Bob