Are you talking about using a tiny 10uF highpass filter feeding the 8 ohm speaker? The cutoff frequency is calculated to be 2kHz but it will acyually be maybe as low as 1kHz due to the inductance of a speaker causing its impedance to be higher.You can't connect anything more than 10u to an 8R speaker on a 555.
The output swing of a 555 into a low impedance with a 6V supply is only 1.5V peak with a max current of 188mA peak. So low frequencies can be used with a huge output coupling capacitor.