I was told that the first e in "leben" or "stehen" or "sehr" are pronounced like the e in "pet (Eng.)" . However, all German singers and speakers seem to pronounce it like the i in "Licht (Ger.)", therefore "leben" sounds exactly like "lieben" to me! What's the correct pronounciation???
The first e in "leben", "stehen" and "sehr" is a long e and thus pronounced like the "a" in "late" without the "-y" sound.
The e in "pet" is a short e that you can find in the last e of "leben" and "stehen".
In no case an "e" is pronounced like "i" in "Licht". In particular, "leben" and "lieben" are pronounced differently. There's normally no danger to confuse them. The singers you are referring to probably speak some dialect. Or, they took some artistic freedom in their pronounciation.
The e in Leben is long, so it is quite different from the e in pet, which, if my understanding of English pronunciation is correct, is more like the short ä in hätte.
The German long e is also different from the German long i as in Lieben. For a German there is no danger of confusing them. If your native language does not have both of these or does not distinguish them, then it can however be hard to hear the difference. English for example has very few long monophtongs and none that corresponds to the German e. This will depend on the dialect, though. According to Wikipedia, a Scottish pronunciation of play can have this vowel. You can also go looking for it in this chart.
Please don't take this YouTube video you mention in your comment as an example. This is a guy who is trying his best but he definitely is a dialect speaker (what more or less all of us are) who tries hard to sound 'standard' (which doesn't exist anyways IMHO - but in lack of a better word).
Just listen to the difference in sound of the two vowels 'I' when he points them out to be exactly the same by his phonetic symbols.
ca. min 3:50 -> Du ähnelst ihm wirklich kein bisschen
Talking about 'wirklich' and 'bisschen'.
-> The two 'i' in 'wirklich' sound very different. The former is a mixture between the german 'Umlaute' ö/ü and the i and the latter is more like the 'i' he explains earlier to be an 'ie' (though a little shorter) - same for the first pointed out one in 'bisschen'...
The most obvious difference in pronouncing german standard HOCHSPRACHE and austrian standard HOCHSPRACHE or however you might call them (let's don't get lost in translation and technical terms) is not bound to political borders (the differentiating terms german/austrian are not chosen very wisely) but it's clearly audible to make a generalization.
'Germans' have their tongues slightly more back in their throats when they are speaking - 'Austrians' (not in the west but the further you go east) more up against their palates and slightly closer to their teeth.
And this is exactly what makes the big difference between the 'ö/ü' sound and the 'i' sound (like in english to see/the sea).
For the german 'e' the tendency is even stronger towards an open short 'ö/e' - Austrians tend to pronounce it more closed without the ö/ü color in it.
The guy tries his best but however you can hear the difference - imagine the real world - there the difference is gigantic and to a sensible ear of a foreigner with more awareness of these diffs of vowels this might lead to confusion.
leben - lieben (written)
leeeben - liiieben (Austrian)
löööbön - lüüüben (German)
Of course - I am shamelessly exaggerating in the orthography to bring across the point ;-) But if you put the vocal track of this video in an audio-editor (or even better in a sampler) and trimmed it to the edges of the vowels, you could play back the sound samples on a keyboard like a D.J. and you would hear
-> ö - ö - ö - ö respectively ü - ü - ü - ü (german)
-> ee - ee - ee - ee respectively ii - ii - ii - ii (austrian)
It takes one word (preferably with e and i in it) to tell if one belongs to the german sounding group or the austrian sounding group. Not two words or a phrase - no diffs in grammar or usage of localisms. Just the sound of their i and e identifies them unmistakable...
If you are curios I collected a few snips of the diversities of our language (written) ->