The speakers in this debacle are always the stock speakers built into my Kia -- I have no idea of their dimensions or specs and it makes no difference here. The only thing that's changing are the droids -- specifically, the operating systems therein.
I know. The initial question you had in this thread was how to make the headset sound as loud as the speakers in your car. In which case, you have to consider the hardware first before the software. From your recent threads, what I gather are:
1. Your new JB phone is not the same model as the GB phone we are talking about.
2. You haven't mentioned wether you are using the same headset on the GB phone.
So here's the things:
1. Even if two phones are on the same OS version, they will not have the same volume for music output as it depends on their internal music chips as well. Case in point my Galaxy S and Galaxy Fit. Even using PowerAmp and 2.3 on both, the Galaxy S volume and quality is superior on headsets. So in your case, it is entirely possible that the headset volume problem is not JB, but the phone you bought. Even on reviews, by GSMArena where they test the decibel levels of music output of phones using a standardized headset and audio equipment sensors, there are sometimes quite significant discrepancies in volume levels on phones with the same OS versions but differing models and brands.
2. You mentioned that on JB, your headset volume appears weaker than before when you were on GB. Now we have to consider that not only do you have a different phone (meaning different audio chip or no audio chip at all, as well as different rendering software), but do you also have a different headset? Not all headsets are created equal. Some may have used wiring with greater impedance than others, which decreases volume and sound quality output. Then you have to consider the quality and size of the earbud audio drivers, as well as the noise isolating capabilities of your headset.
Now you keep on pushing that you feel JB may be the culprit in your headset debacle, but I just listed down three very important factors which may affect your headset volume, and none of them have anything to do with JB:
1. Model/Make/Hardware of phone
2. Brand (software optimizations)
3. Headset.
Examples of real life applications:
-Model:
2.3 Galaxy S vs 2.3 Galaxy Fit = GS is louder and better quality
4.2 Galaxy Tab 3 (JB) vs 4.3 Galaxy Note (JB) = Note has better quality at higher volumes
4.3 Galaxy Note vs 2.3 Galaxy S = Volume is actually louder on the Note (in exact opposite of your debacle)
-Brand (I have all Samsung Androids, so meh, see online tests)
-Headset
Using the Galaxy Tab 3's headset with no noise isolation, I need to put the volume up to 50% to have an equal output to 20% volume using the ear-plug type isolating headset that comes with the Note 2.
So basically, unless we get more info from you, it cannot be concluded that JB is indeed the problem with your headset