How can I measure the brightness of a LED?
It's not clear if you want to actually encode the voice signal with LED brightness, or whether the brightness is just supposed to indicate recent amplitude.  In any case, none of this requires actually measuring LED brightness, so it's not clear what you are really trying to do.

The simplest means to measure brightness is probably with a light dependent resistor (LDR).  These are CdS cells that vary quite dramatically with light intensity.  The signal is strong, but CdS cells are relatively slow (good for voice amplitude, but borderline for actual voice), and non-linear.

Light can also be measured with photodiodes or phototransistors.  These are faster and can be arranged to produce a linear signal.  But, they are a little more complicated to drive and their signal is generally "small".  They require active amplification to get to typical useful levels, and therefore are subject to picking up noise.

There are two broad ways to use a photodiode, in leakage mode and in solar cell mode.  In leakage mode, you reverse bias it.  The current is reasonably insensitive to the bias voltage over a wide range, and quite linear with light.  It is small though, so is usually run directly into a transimpedance amplifier.  In solar cell mode, the diode is kept shorted, and the current resulting from it acting like a solar cell is measured.  This produces good results, but usually requires a negative supply to make a positive voltage.