Capacitance value is only one of two variables that are likely to change over time for electrolytic caps such as those caps in your pic. The other variable is equivalent series resistance (ESR). Generally, ESR increases over time, making the speaker sound like it is muffled by a heavy blanket when it gets really bad. ESR is also hard to measure at audio frequencies unless special equipment is used. Most inexpensive ESR meters measure at a frequency well above the audio range, and since ESR varies with frequency, measuring above the audio range is useless.