In Czech pronunciation there are pairs of consonants, one consonant of the pair is voiced, one is unvoiced:
ch-h (this pair has actually three members, but I don't want to overwhelm you)
There is also voiced and unvoiced "ř."
There are also consonants that stay single and are voiced (j, l, r, m, n, ň), but don't - mostly - interact with others 😶
In certain situations, pair-consonants are pronounced as their opposite ones. The "strong" one can be weakened, the "weak" one can be strengthened.
In Czech, this happens mostly backwards - so it depends, what's at the end of the group of consonants (what is written below doesn't involve the unpaired/single consonants, they don't do this):
1) if the word ends by a consonant and there is nothing following (like when you end a sentence or say a single word), it's pronounced as unvoiced by default (many native speakers don't know this and don't believe it, but it's true, they just have in their mind a strong connection to the fact that they know it's supposed to be a voiced consonant)
2) if there is a vowel following a consonant inside a word, it mostly preserves its basic nature and stays unchanged, but there are weird situations when this happens between words...
3) if the group of consonants ends with voiced one, all of the members of the group are pronounced voiced
(but if the by nature voiced consonant ends the word and it's weakened by the rule described in 1), it behaves like unvoiced! - e.g. a word "vjezd" is actually pronounced "vjest" if it stays at its own).
4) if the group ends with unvoiced consonant, it whole ends up being unvoiced.
As always, there are exceptions (or... abnormalities ):
a) "ř" is the only one that is also affected by the previous consonant, so in "před" and "břeh" it's actually pronounced differently, but it's a thing even native speakers ignore, the difference is very small
b) certain dialects have their own rules how the pair consonants behave: most noticeable ones are Moravian tendencies to use "single" consonants to strengthen previous consonant ("k nám" pronounced as "g nám") and Silesian custom to pronounce "kv" as "kf" ("kvůli" as "kfůli").
c) in parish books esp. in German, there is a new level of problems introduced by the fact that German has a different concept of voiced and unvoiced sounds, so even in Czech "strong" positions, they may feel it unvoiced (and vice versa).
d) someone may just ignore everything and use whichever consonant of the pair he considers to be fitting.
Everything described here is about pronunciation issues, that are now mitigated by a language codification of written Czech, but in case of old uncodified language and different languages involved, it creates quite a mess.
(Remember that in old times, they mostly tried to record what they heard, but sometimes they also decided to respect the actual word and/or even overcorrect it, which may cause different spelling).
There are more changes in consonants of different origin, but for a basic conclusion of this particular one, you need to remember, that if you have a pair-consonant in the surname (or name of the village) and you struggle to find something, it could be simply switched to the other consonant of the same pair.
It's also important for using old indexes, as the surname may therefore occur under a different letter or the pair-consonants may be grouped together.