As a matter of fact, the letter "á" is not used in French. Diacritics only affect the quality of the vowel — that is, the "place" in the mouth where the sound is formed — not intonation, tone, stress or anything like that. (In certain dialects, diacritics may also indicate the duration of a vowel.)

The letters with diacritics that change the vowel sound in French are è é ê ô and for some speakers â.

Accents on E

An accent mark on E indicates that it is not pronounced as schwa (ə), but as a front unrounded vowel of some kind (that is, a vowel in the area of IPA [e] or [ɛ]). The exact number and distribution of non-schwa E sounds varies between dialects/accents of French.

• É usually corresponds to the sound /e/, called "close e". This sound doesn’t exist in English, but it’s like an /ɛ/ that is pronounced with the tongue slightly higher in the mouth. Some people find it helpful to think of it as the sound in "grey", but with the y-like sound at the end removed or smoothed out. It is a pure vowel sound, unlike the "ay" sound in English. I said "usually" because the phenomenon of "vowel height harmony" causes some speakers to use [ɛ] instead in some words where a low vowel is in the following syllable. See Eau qui dort’s comment here for an explanation of this.

• È is pronounced basically like the vowel sound in English "bet", called "open e", and transcribed /ɛ/ in the International Phonetic Alphabet. I can’t think of any exceptions to this. In some accents, it apparently may represent a long open front vowel, /ɛː/, in certain words (see the next bullet for more discussion of this; I first learned about it from James’s question Pronunciation of “freiner”).

• Ê is typically also pronounced as an open e. The digraph aî makes the same sound. In some accents, the letter Ê represents a distinctively long vowel /ɛː/, but I believe this distinction is lost for many speakers nowadays. I was never taught to do so, but as an example, some people pronounce "maître" with a longer vowel sound than "mettre". As with É, "vowel harmony" effects cause Ê to have a different pronunciation for some speakers in certain words where it is not in the last syllable: for example, the word "bêtise" can be pronounced with a close e in the first syllable (/betiz(ə)/) because of the high vowel /i/ in the last syllable.

The circumflex accent on O or A

• The vowel ô is pronounced like the digraph "au": a close o sound, IPA /o/, that has no exact English counterpart. (For American English speakers, it is a bit like the "oh" sound smoothed out to make it a pure vowel sound. For British English speakers, the vowel sound of "aw", as in the word "law", may have a similar quality, as aCOSwt pointed out in a comment.) In accents with vowel length, this is a long vowel.

• The letter â can be pronounced on various ways: in Parisian French, the tendency is to pronounce it the same as an unaccented letter a, with a sound a bit like that in the English word "cat". In more conservative varieties of the language, it has a sound like the sound in English "father". It is also a long vowel in accents with vowel length.

Other accents don’t affect pronunciation as a rule

A circumflex accent can also appear on the letters î and û, but it is not generally associated with a different pronunciation. Some speakers have a different pronunciation for the digraph eû compared to the digraph eu; as far as I know the only words where these contrast are jeune /ʒœn/ and jeûne /ʒø:n/ or /ʒøn/ or even /ʒœ:n/. But this distinction is not universal even among French speakers.

The letters à and ù have the same pronunciation as the unaccented letters, but they are used in the spelling of some common words to differentiate homophones. The most common example is the preposition spelled à vs. the form of the verb avoir spelled a. (The only word spelled with ‘ù’ is the word où ‘where’, to distinguish it from ou ‘or’.)

You might also see the trema mark, two dots above a vowel, that shows that the vowel must be pronounced separately from a preceding vowel. For example, the word naïve has two syllables, so it gets a trema on the ï.