Your circuit has so many things wrong that its voltage gain will be very low and you must scream at the mic for the LED to flicker.
A The electret mic should not have R10 in series with it.
B R29 should be 4.7k ohms.
C C8 should be a 1uf non-polarized film capacitor.
D The transistor should be a high gain small signal transistor like a 2N3904.
E The transistor should have an emitter resistor for DC stability and a bypass capacitor acroiss it.
F The base bias resistors R23 and R24 should be calculated to provide enough base current for the transistor.
G R4 should be about 180 ohms for the LED to be bright.
H A second transistor is probably required.
Thanks for the reply.
Concerning A, that resistance is to simulate the impedance of the mic. The characteristics can be found in this page (they are in Portuguese):
**broken link removed**
B, you said that the mic should be fed with 500uA but I only had 2k resistors at home =/
C, Only had 47u, 100n and 10n.
D, how can I know what kind of transistor should I use?
E, I actually had that in the previous schematic but since it wasn't working I started removing stuff and leaving only the basic. I forgot to re-add it.
F, how much current is "enough current"?
G, with the circuit designed as it is now, by lowering the R4 value the LED stays always on. A tried with 470Ω and the LED stopped blinking. I also remember from class that the gain is related to the Resistance connected to the Collector, in this case, R4
H, I have another TIP31C right here =P
Thanks for the help