According the site rules, answering your own questions is encouraged to provide a useful resource to the community, so here goes (I hope someone finds this useful....).

After my initial edit when I found that SO₂ absorbs in the UV only, I continued to do more reading, and think I have found reasonablely satisfactory answers to 3 out of my 4 questions, which were:

1. What would the effect of sulfur vapor humidity in the air be on visibility? Specifically, what partial pressure of vapour seems reasonable? (I can approximate from the quote above from Freitas)
2. How far will light penetrate through the sea?
3. How far and how well will sound travel through the sea?
4. How effective would detection of electric fields be in a sulfur sea? (E.g. for prey detection, as in sharks)

I'm still not sure how to address the humidity question - I might have to just guess since som many variables could affect it.

My answers so far are:

2. How far will light penetrate through the sea? 
 In his answer, HDE 226868 found values for the optical absorption coefficient in liquid sulphur (thank you very much!) - a range of 5.5 x 10^-4 m^-1 at 130°C to 2 x 10⁵ m^-1 at 450°C. However the variation in absorption coefficient over this range will not be linear as he assumed - liquid sulfur is light yellow up to 157°C where it becomes dark red, and then at 225°C it becomes black. Clearly the variation in absorption will experience large jumps at the transitions between the three forms. The maximum absorption of 2 x 10⁵ m^-1 at 450°C corresponds to the black form; the lower bound of 5.5 x 10^-4 m^-1 at 130°C is just below the middle of the temperature range of the light yellow form (113°C - 157°C).

In my question I specified a temperature range of 120°C to 170°C for the planet, with most oceans and rivers in the yellow form, so it's reasonable to take the 130°C figure as a rough guide - 0.00055 m^-1 .

For comparison with water, absorption coefficients range from: 0.0044 m^-1 at 418 nm (violet) 0.02 m^-1 - 500 nm (green) 0.2 m^-1 600 nm (orange) 0.6 m^-1 700 nm (red) 3 m^-1 740 nm (red)

The absorption coefficient at 130°C is an order of magnitude smaller than the absorption coefficient of the least absorbed wavelength of light (violet). However, liquid sulfur's color clearly demonstrates that a narrower range of frequencies get through (i.e. we can assume that absorption coefficients for other frequencies are much higher than they are for water), so perhaps overall illumination would not be as bright as the lower absorption coefficient would suggest. It seems reasonable to assume that yellow light will penetrate a sulfur sea less deeply and illuminate less brightly than is the case for sunlight in seas on earth, but not so much less that vision and photosynthesis in surface dwelling species would be unreasonable (the photic zone would just be shallower). This does not take into account stronger 'sun'light from the planet's star - I can handwave that a little to reduce the differences further.

Overall, I consider a reasonable answer to my question to be that the seas will be yellow and darker than our seas, but not so much so that it should make vision or an oceanic ecosystem drastically less viable than on earth."

3. How far and how well will sound travel through the sea?

Thanks again to HDE 226868, for the information about the speed of sound!

According to the majority of sources I found while google-trawling, absorption losses are minimal compared to the inverse-square law losses. At low frequencies especially, losses due to viscosity are pretty low.

**I consider it reasonable that sound will travel reasonably well and not that much differently to in earth's oceans, and so echolocation and sound communication would be reasonable.**


4. How effective would detection of electric fields be in a sulfur sea? (E.g. for prey detection, as in sharks)

I hadn't noticed at first, but the aforementioned Freitas book actually contains a value for the relative permittivity of liquid sulfur - 3.48. Water has a typical relative permittivity of 81.1, which means that an electric field at a given distance from a charge would be 24 times stronger in liquid sulfur than in water (since electric field strength is inversely proportional to permittivity). This is not what I was expecting and I am a little concerned I have had a small attack of cerebral flatulence while working this out, but it seems to be correct. 
 It would seem that detecting electric fields would be a very viable option for animals living in the darker bottoms of a yellow sulfur sea

If anything is obviously wrong with these answers, please point it out!