Is the voicing voiceless consonants common in the US?
I don't know if I should trust my non-native ears, but I've heard a couple of people (Katie from CollegeHumor is the first one come to mind) who say "thank you" with a voiced "th" instead of the proper voiceless one. Also quite a couple of people seem to have said "princible" instead of "principle" (like those guys from Corridor I think). I don't remember the exact videos where they say those words for demonstration, though.
So do Americans often voice their voiceless consonant if at all? Discounting intervocalic voicing which is more of a historical thing that happened centuries ago ("knives, clothes, Stephen"), this seems counter-intuitive to me, because it's more natural to devoice voiced consants, especially at utterance boundaries or if that consonant is next to a voiceless one (e.g., "slide" + a pause, in which /l/ is slightly devoiced because of /s/ and /d/ slightly so because of the following pause). Also discounting the North American tapped /t/.
No, it isn't that common to voice voiceless consonants. It does happen sometimes, though (the biggest example of course is intervocalic /t/ in certain contexts).
The two examples that you give are quite specific.
Voicing the initial th in thank is known to be a feature of some people's speech (See Voiced “th” in “thank you”? on Linguistics SE and Pronunciation of "thank" using ð (voiced th) instead of θ (unvoiced th) on ELU). The distinction between voiceless and voiced th does not have a large "functional load" and it isn't indicated in the spelling, either.
I hadn't heard before about voicing the p in principle. It seems possible that this is partly an analogical change based on the large number of words ending in -able/-ible—although those are mostly adjectives, while principle is a noun. Based on Google results for the spelling "multible", the word multiple may be another word where some people voice /p/ to /b/ when it comes after an unstressed vowel and before word-final /əl/. I haven't heard of any analogous change of voicing /kəl/ to /gəl/ in words like miracle, article, vehicle, oracle.
In some words, sibilant sounds may be unexpectedly voiced.
KarlG mentions the use of voiced /dʒ/ in congratulations. Some speakers use voiced /dʒ/ word-finally after an unstressed vowel in certain words where /tʃ/ would be expected from the spelling, such as ostrich, sandwich. I don't know if this /dʒ/ is actually phonetically voiced, or just phonemically voiced. Actually, the variation between /tʃ/ and /dʒ/ after unstressed vowels seems to date back to Middle English, so it didn't originate as a specifically American English thing (it's discussed in §4.2.1.5 of "Change in Obstruent Laryngeal Specifications in English: Historical and Theoretical Phonology", by Magdalena Jeannette Spaargaren, 2009).
Voiced /ʒ/ is used where /ʃ/ would be expected in a number of words, such as equation, words ending in -rsion (version, dispersion, immersion), and fission. The pronunciations with /ʒ/ have varying frequencies for each word depending on the dialect. Using /ʒ/ in -rsion words seems to be a specifically American English thing, while using /ʒ/ in equation is usual in all major varieties of English as far as I know.
The distinction between /s/ and /z/ is relatively unstable in certain contexts/words (interchange between these is probably facilitated by the use of the letter S to represent both sounds). The final consonant of versus is often voiced (even by speakers who have not reanalyzed the word as an inflected form of a verb "to verse").