I view the Audio Response as one of those pieces of functionality that allows the user to most accurately "duplicate" the type of sound they may have gotten use to with previous detectors. If you've been hunting for several years, you've probably had at least one detector that you wish you still had, just because of the sound it made when you struck pay-dirt! If the E-TRAC is one of the first detectors you've owned, then it is just a matter of what sound suits your style. As someone who has owned more detectors than they care to admit, here is my take on it......
Most of my detecting is for old coins at old sites. Typically old farm fields where homesteads or Fair Grounds use to be. In those areas, I use Normal at least 90% of the time. TTF in areas with an abundance of iron and MTC in areas with small amounts of iron or an abundance of "modern" trash. I know the book says that "experienced users" could benefit from Long tones. If that makes me a "beep and dig" kind of guy, so be it. But I like hearing each target signals start and stop, with a break or short burst of Threshold in between. And if I control the coil speed for the specific site, Normal tones provide me with all the audio information I need to determine when to dig. I don't view the "break" or "null" to effect my "reset time" any more than when the audio pitch has to change for each target. If it does, then I'm sweeping too fast anyway.
To me, the continuous sound of using Long tones makes me think too hard.

If they work well for the "experienced users", good for them. In the places I hunt, I just don't feel Long tones are beneficial if I listen carefully to each target, can hear each one's signal "start and stop, and slow down my sweep accordingly. If the area I am hunting is extremely congested with targets and separation is the issue, a small coil and slow sweep will separate those targets better than any audio response I could select. With that said, there are times when I've had some success using Long tones. But in those instances, I didn't use Long tones to decrease my "reset time". I did it because the I was hunting an old fair ground that is filled with nails, shotgun shell casings and an occassional coin. Hearing the differences between iron, shotgun shell casings and coins in MTC is much different than listening to an abundance of modern trash that produces tones similar in pitch to an old coin. In those sites, I never use Long tones or TTF.
I don't like Smooth tones because the variability of the pitch is "rounded off". I set my Variability to 28 to hear a wide range of pitch, and feel it is "wasted" if I reduce the effectiveness by using Smooth tones.
Pitch Hold does not provide me with the target separation that I believe I can get from Normal. If I want to hear the highest pitch over a target that I think might be good, I'll use Normal to locate it and Pinpoint to isolate it. I don't need a continuously changing tone to confuse the issue.
Audio tones are like many of the other "options" offered on the E-TRAC. My advice to you is to find the one you best interpret, and stick with it until you have it mastered. JMHO HH Randy