CT,
What no one has asked yet is the distance that you need to run these wires? It's been mentioned that shielded cable cost more money (which it does), but if you're trenching 2ft deep for more than 20-30ft (you may be going hundreds) the cost of labor and tool rental if your not doing it by hand would far outway this.
Sandpiper's given you some good advice... 240v has the "potential" to cause more interference as it's usually only run for devices which require lots of current. There's really no difference that I know of between 120/240 as far as EMI is concerned, it's all determined by what's plugged in at the other end.
As InsanePhotoGuy mentions, you should have concern if that CAT5 was going to be used for high speed data, but your weather station is likely RS-232 or RS-485 so shouldn't be a problem with proximity to the High Voltage. Same with the speaker wire... you'd need to place speaker wire right next to high voltage for some distance to get any interference (and this provided there is a high current, electrically noisy device on the high voltage line).
Here's the advice I would give (most of which is just sumarizing what the others have posted:
-Do use PVC conduit
-Do use "weather-proof" cable for both the CAT5 and Speaker wire. This can be different than "direct burial" cable which will always have a tougher and UV resistant jacket, but may not be gel filled which is what's important for your use. Check the package because companies mark/market cable differently... ie durect burial vs weather-proof.
-Size your speaker wire accordingly to the distance from amp to speaker. 16g is good up to around 100ft, 14ga will get you around 200ft, anything longer you should use 12ga.
-If you're unsure if/where any utilities are buried, call to get them marked. Especially if you'll be using something mechanized to trench like a Ditch Witch.
-If the run is longer than 300ft, you might want to consider installing a vault somewhere along the middle to act as a pull point which will make it easier to pull the wire (high and low voltage).
-Don't forget to install pull strings while putting your conduit together.
-Use large enough conduit to make the pulls easy and to leave room for future low voltage wire if needed.
-Always pull a new string in with the wire so you can leave one behind for future use.
This is all fresh in my mind as we've just burried ~1400ft of 2" conduit for a customer job to get Cable TV and Phone from the road to a new house, and intercom & gate control from the house out to the front gate.
Cheers,
Paul