If the circuit pulls more current than the source wants to give, you will get a voltage drop.
Even too many leds can take Arduino 5V down to 4.5V. At 16 MHz, any lower and it may get unstable.
So what happens when the sound chip and SD are both at full pull? A few 10th's of a volt may help.
I'm pretty sure that I can feed SD 3.7V and likely most other 3.3V devices as well.
FWIW, Arduino can take 5.5V too, but you probably shouldn't put 6V on VIN.