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The 1.8V is likely the Multimeter’s representation of the signal bouncing between 0V and 3V very fast and the meter is showing the RMS(aka average) value of it. Placing Serial.println(F(“!”)); of very short/unique messages may help you identify what is actually happening. I often put the Serial.println(F(“.”)); in the libraries SFEMP3Shield::refill(). Noting that refill happens so much, that long messages can overflow and cause problems. Hence short messages, just enough to give you insight of where it is locking up.
Test by omission the INT1, what are the results after changing SFEMP3ShieldConfig.h
from
#define USE_MP3_REFILL_MEANS USE_MP3_INTx
to
#define USE_MP3_REFILL_MEANS USE_MP3_Polled
Finally, there is a remote possibility that it may be the SdCard not working reliably, causing everything to back up.