Faulty speaker or microphone
A smartphone actually has several separate microphones and speakers (call earpiece, media speaker, noise-cancelling mics) - finding which one is at fault changes the diagnosis a lot. Dust built up in the grilles is the most common cause, and the easiest to fix.
FAQ by brand
Apple (iPhone)
Sound is muffled only during calls, not with music?
That means the call earpiece (the top speaker, separate from the bottom media speaker) is dirty or damaged, rather than a general sound problem.
The other person hears me badly while I hear them fine?
The problem is probably one of the microphones, not the speaker. Several mics take part in noise cancelling, so a single faulty one can blur the voice you send without affecting what you hear.
Samsung
How can I find which audio part is at fault before a repair?
The hidden diagnostic menu (*#0*#) tests the speaker, the earpiece and the microphone(s) separately, which avoids replacing a part that actually works fine.
Xiaomi, Huawei
The sound crackles only at high volume?
That's a classic sign of a damaged or dirty speaker membrane - a professional cleaning is sometimes enough, otherwise the part needs replacing.
Other brands
Can water exposure cause this kind of fault, even on a “water-resistant” phone?
Yes - water-resistance certification (IP) wears down with time and knocks, and speaker grilles are a classic entry point for leftover moisture that affects the sound afterwards.
Your problem isn't listed here?
Every device is different - ask for a free diagnosis or describe the problem directly on WhatsApp.