I am going to use AVR's PWM output signal to pass through a low pass filter (maybe plus an amplifier). I found the speaker has a quite low resistance (8 ohms). I tried to simulate my schematic on Orcad, obtaining very a low voltage signal on speaker in (mV). I think that is the problem of this load resistance (I simply put the load resistance across output terminals). But I am not sure how to deal with that. Hope someone can help. Thanks in advance. The same problem occurs when I tried using single simple pole low pass filter. Looking forward to hear your suggestions.
Your speaker rated at 1W will require an amplifier circuit to be placed between the DAC / filter circuit and the speaker. There are numerous solutions to provide for this amplifier function including discrete circuits, IC circuits and single chip amplifier modules. You could build up a circuit using a chip amplifier such as an LM386 or you could check online sources to get a low cost ready-made circuit module. Sellers on eBay are making an assortment of modules available such as this example: