Hi Franco,
If you have no way to take an aocilloscope in the field, then your filter circuit to detect 3-30 hz can help see the lower end of the spectrum. You should also be able to hear the spectrum from 30-1200 hz on the speaker or headphones.
Here is another Idea: Ther are low-power cmos phase locked loop chips that will follow a frequency, and can be used in conjunction with a cmos decade counter that could change the frequency of what you sense on the signal. The decade counter can take the frequency of the input signal and divide by 10 to allow you to hear signals in the 12,000 to 120,000 hz range, or cascade a second counter for higher frequencies. The input signal to these counters would be sent through a schmidt trigger first with an adjustable threshold to set the sensitivity. I believe the cmos PLL has both an analog and digital input, and the analog signal threshold can be adjusted with trimpots.
These decade counters can also be used to multiply the low frequencies to 10 times higher, so they will produce an audible tone. This is done by using the phase locked loop's oscillator to clock the counter at a rate 10 times faster than the antenna signal, then using the carry from the counter to sync the PLL frequency. The only problem with this frequency multiplier scheme is the amplitude of the signal is lost after the digital stages. Perhaps the amplitude could be recovered by putting the digital signal and the filtered antenna signal into a comparator. The digital signal will have a known voltage, while the filtered antenna signal can be compared and the output could set the gain on the audio amplifier.
If any of this circuitry was to be built, then It would look like a control knob that you adjust to several positions: F/10, Fx1, Fx10, Fx100. Similar to sweep time constants on an oscilloscope, while you listen for the sounds.
It seems to me that a meter is ok for a simple indicator of signal. Audio will tell a lot more information because you can hear frequencies and amplitudes. But oscilloscope is still the best because you will see many things that cannot be heard, and you will be able to enlarge any small detail that you want to see closer. I have seen some small portable oscilloscopes on ebay. Some of the older ones seem like they sell cheap, and can be connected to an inverter at your car battery. Here is an example... maybe there are inexpensive deals like this in your area...
http://cgi.ebay.com/Oscilloscope-BK-...QQcmdZViewItem