Both LEDs and speakers are driven by current; however, LEDs only work with
current going one way, and further have a minimum voltage requirement of
around 1.5V.
Running your sound card at full volume will cause it to put out more power,
which will increase the LED brightness. However, transformers don't increase
power, they just change voltage and current. Thus, since power is equal
(ignoring losses in the transformer) you have I(in) X V(in) = I(out) X
V(out). Therefore, increasing V with the transformer decreases current in
proportion.
If you are driving 4 LEDs with this, The voltage must be greater than 4 LED
drops, which is about 6V. Since the voltage gain of your transformer is
about 11, and you couldn't see the LEDs light before using it, I'm guessing
that your sound card signal is between .6V and 1.5V peak to peak. Normal
LEDs light up with about 1mA of current, so your 11 gain means your original
speaker current might be around 11mA. Thus, if you got a transformer with a
smaller turns ratio, (ie, 250 ohm to 8 ohm, for example,) the current would
be twice as big, but your voltage would be 1/2. This would mean you could
drive fewer LEDs, but you would drive them more brightly.
Note that the current through the LEDs will be the same if they are in
series, which is why they all light with the same intensity.
You could also try a rectifier diode ($1 at RS) to put more of the voltage
on the positive side... that will ensure that the current is travelling in
the right direction for more of the cycle, thus causing the diodes to light
up for twice as long (they are flashing now with the frequency of the sound,
and only lighting when the voltage is positive). However, there is a voltage
drop across these rectifier diodes of around 1.2V that you'll need to take
into account. Put the AC leads (the ones with a tilde marking) across the
speaker wire outputs, and connect the long lead of the LED to the + end of
the rectifier, and the short lead to the - side.
Disclaimer: I'm a hobbyist, not an EE. You are on your own if you follow
this or any other advice I give you...
Regards,
Bob Monsen