Silence substitution (don't)
Replacing the lost packet with silence is the simplest fix. It produces audible clicks at packet boundaries (energy discontinuity). Acceptable only if loss is rare (<0.1%) and packets are tiny (<5ms).
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Last-packet repetition
Repeat the previous frame in place of the lost one. Cheap, mostly transparent for vowels. Audible 'metallic' quality for music or fast transients. Standard fallback when nothing better is available.
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LPC extrapolation
Linear Predictive Coding analyzes the previous 20-30ms, fits an AR model, extrapolates forward to synthesize the missing frame. Used in G.711 PLC and Opus. Better than repetition for both speech and music, especially across consecutive losses.