⟨ch⟩ is pronounced [x] in Polish and as many other Polish sounds, it can undergo so called "voice assimilation".
Assimilation is a process during which a speech sound gets a feature from an adjacent or otherwise close speech sound. Typical example of the is the voice assimilation, causing that the voicedness of a consonant depends typically on the consonant directly following it, e.g.:
I have a dog - [aj hæv]
I have to go - [aj hæf tu]
Most typically, this process is limited for a class of sounds called obstruents, i.e. sounds formed in a manner where the articulatory organs create an obbstruction (in case it is full, they are called occlusives, in case it is partial, they are called fricatives), through which the air is expulsed and creates a noise (non-harmonic sound).
Thus the obstruents are sounds like [t,p,k,d,b,g,f,s,š,x,v,z,ž,ɣ,...] but not [m,n,l,r,...].
In Polish, this works in a very similar name - when a voiced obstruent follows an unvoiced one, such as [x], the unvoiced one is assimilated and becomes voiced, in this case [ɣ], even though otherwise there is no stand-alone [ɣ] in Polish. This does not happed in front of liquids (l, r) and in front of [v] (which behaves like an approximant in Slavic languages)
śmiech siostry (laughter of the sister) - [ɕmjex ɕostry]
śmiech brata (laughter of the brother) - [ɕmjeɣ brata]
chleb (bread) - [xlep]
The same rules should apply to [x] sound regardless the way it is written - be it ⟨ch⟩ or ⟨h⟩. The best strategy to pronounce this correctly is to pay no attention as the voice assimilation is very widespread process and most speakers do not even realise this is happening (especially since [ɣ] is not a phoneme in Polish, i.e. it does not distinguish meaning).