I can tell you without doing the sweep that it won't be what you expect. VLSI (the mp3 decoder manufacturer) indicates that:
The analysis of low frequencies is not very accurate in the VS1011 version.
But overall, I've found that you get a pretty good representation of what is going on. Also, when you boost up the serial connection to the rMP3 up to 115200 bps using a hardware serial port, you get more samples, and it responds more smoothly.
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