Headphones are usually 32 - 64 Ohms, so with a 2K series resistor you should see a significant signal loss, so the filter is not going to work right with headphones.
1500Hz is not "bass", but a 1500Hz low-pass filter should sound VERY "muffled".
You didn't tell us the capacitor value (and a schematic the best way show a circuit).
The line input on an amplifier, or the input to powered computer speakers is typically 10K - 100K and the Arduino basically has infinite resistance ~100M Ohms so it should work as expected with those loads. (although you can't leave the Arduino input "floating" and you'll need the bias circuit so your impedance/resistance will depend on your circuit.)
[u]RC Filter Calculator[/u] This assumes a high-impedance load (relative to the resistor value).
I see, I have built this low pass circuit, tho I don't know if it works, I dont have an oscilloscope nor a multimeter.
But here's what you CAN DO. Generate some tones at high (or mid) frequencies with your soundcard.* Then read the analog input with the Arduino and send the results to the Serial Monitor. Try it with and without your filter. (Of course, you'll need to find the peaks, etc., as I mentioned above.)
And you might consider buying a meter. A cheap meter is better than no-meter, but make sure it can read low AC voltages.** They are super-helpful for tracking-down problems. If this is your one-and-only electronics project, MAYBE you can get-by without one, but if this going to be your new hobby, I strongly recommend you get one.
I don't have an oscilloscope at home, but I have access to them at work. Sometimes I "lust" for one, but so far I haven't needed one. And if you do get a 'scope, in this case I'd recommend a "real" benchtop scope, NOT some cheap little built-it-yourself thing or a board that connects to your computer.
- If you don't know how to make test-tones, [u]Audacity[/u] can generate sine waves that you can play immediately or export-as an audio file.
While you're at it, generate and listen to a 1500Hz tone and some other frequencies. Most people would describe 1500Hz as "high pitch", although technically it's closer to mid-frequency (on a logarithmic scale) and you can hear a LOT higher.
Audacity also has Equalizer & High & low-pass filter effects if you want to experiment hear what filtered music sounds like.
** My meter is not super-cheap but it doesn't read much below 1V on the AC range (at least not accurately). A lot of audio signals are lower than that, so someday I'm going to upgrade my meter.