Calls sound fine.
The built in speaker is surprisingly loud and clear. However most of the time when I'm playing music or podcasts I'm in my car with the phone plugged into my radio. I don't have any problem getting enough volume to hear the song or story - which is saying something considering I'm driving an old Miata. So the phone has to compete with the typical sports cars exhaust note and the general loudness that one has to deal with when driving a convertible.
I've not spent enough time with videos to have a strong opinion. The few that I've watched are fine.

 

audio on videos SUCKS

youtube audio is fine

but when you load your own videos (mp4), there is a huge lag every single time. I can't stand it.

 

I have never had a lag on a video ... its been right on and clear for every video I have played.

 

I thought so too at first, but you can adjust it. Just use the volume rocker on the side of the phone, and you can make it much louder. I can watch Dr. Horrible's Sing-Along Blog, and it sounds great

 

This wouldn't help the lag in audio.

 

Now that I am here with all my knowledge there can actually be an intelligent response in this thread...

I listen to mp3s and browse the internet while on the train. Playing mp3s does not effect speed. I even checked CPU usage while playing mp3s and it was only a few % which was probably from system use. It looks like the qualcomm chipset has a hardware MP3 decoder so dont worry about it.

 

Same with me. I love my Eris but it drives me crazy that I can't get the audio and video to match perfectly when I convert movies. My brother has the same issue with his. We've been able to get close enough to make it watchable but never perfect. Right now it's the only gripe I have with the phone.