Is the consonant [b] always voiced across languages? What about [p] being voiceless? Similarly, is [k] always voiceless across languages?
Basically, I am taking what I know in English and wondering if it applies more broadly.
It depends on what is meant by "[b]".
Usually, a transcription in brackets means that it's a phonetic transcription, which aims to represent the sounds uttered without considering whether the language makes a phonemic distinction between them.
Under the strictest interpretation, [b] is voiced by definition, since that IPA symbol represents the voiced bilabial plosive. In other words, the "cardinal" or prototypical value of [b] is voiced.
But more commonly, phonetic transcriptions can also be broad, meaning that they notate only the most salient features, or those that are likely to be linguistically significant. For instance, one can transcribe the American English word buddy as ⟨[ˈbʌɾ.i]⟩. This is clearly a phonetic rather than phonemic transcription, since it indicates the specific allophone ([ɾ]) of /d/ that gets used. But it is a broad transcription. In English, utterance-initial voiced stops are usually partially devoiced, which is rarely noticed by native speakers. So this transcription is using ⟨b⟩ to denote a partially-devoiced phone.
Of course, even in a narrow (precise) transcription, ⟨b⟩ may be used to represent a voiceless phone when it is used with a diacritic. For instance, one might transcribe the Russian word го́лубь (gólubj) as [ˈgolˠʊb̥ʲ] to indicate that the final consonant, though phonetically voiceless, is an underlyingly voiced sound that is devoiced by a phonological process.
In a phonemic or morphophonemic transcription, the symbol ⟨b⟩ can easily refer to a voiceless stop. Many languages do not distinguish between voiced and voiceless stops. For example, Korean has a series of stops that vary between voiced and (tenuis) unvoiced. Authors can somewhat arbitrarily pick which allophone to use to transcribe each phoneme; in this case, one might pick the voiced IPA symbols to help distinguish them from Korean's 2 other series of voiceless stops. Then the word ⟨불고기⟩ would be phonemically transcribed as /bul.ɡo.ɡi/, even though the initial consonant is voiceless.