Learn French: Le mixage coûte cher, alors baissez le volume de peur de saturer. - The mixing is expensive, so lower the volume for fear of clipping.

In French, certain adjectives are used directly as adverbs with specific verbs. With coûter (to cost), we always use the adjective cher to say something "is expensive" or "costs a lot". When an adjective acts as an adverb like this, it is invariable, meaning it never changes form. Even if the subject were feminine or plural (for example, les machines coûtent cher), cher remains masculine singular.

Why use de peur de + infinitive instead of de peur que?

You use de peur de followed by an infinitive when the subject of the main clause and the dependent clause is the same. Here, the person lowering the volume is the same person who might cause the audio to clip. If the subjects were different, you would need to use de peur que followed by the subjunctive—for example: baissez le volume de peur que l'ampli ne sature (lower the volume for fear that the amp might clip).

Is saturer the standard word for "clipping"?

Yes. While saturer literally translates to "to saturate" in general contexts, in audio engineering and music (as indicated by mixage and volume), it is the standard French term for a signal becoming too loud and distorting, which we call "clipping" or "peaking" in English.

Could I use donc instead of alors in this sentence?